i have a holley 750dp(4150). it has blue accelerator pump cams on bolth sides and 31 shooter on bolth sides. my problem is that when i stab the gas alittle the carb sounds like its backfiring. the best way i can discribe the sound is, it sound like some one is kickings an empty soda can around. its not a really big backfire. im really not sure what it is. if i rev the motor really easy and slow it won't due it. it only happens when i stab the pedal. i tried messing around with pump cams and i got know were. it was working fine i thought it was timing, but i was messing around with that and it didn't due anything. it sound like it coming from the carb. i checked my fuel pressuer gauge and it was at 5 1/2 psi. then i checked it later on and it was at 3 and dropping. i let it sit then checked the pressure again and it was at 5 1/2 psi. the pump is a mechanical holley pump that is about a year old. the noise still happens at both of the pressure reading. i checked my plugs and they were ashy black but the ground electrode was a mild chocolate color. thanks for any help

 

To me it sounds like it's lean. If the preasure slowly drops it may be a sign of vaporlock.

 

how do i stop vapor lock from happening

 

Are you sure it's backfiring and not detonation? Detonation sounds like a bunch of marbles in a Coffee can when you step on it hard. You've got a fairly small cam for that motor. A small camshaft and high Compression Ratio are a sure scenario for detonation. ( Small cam raises dynamic compression ratio compared to one with more duration and overlap )

What is your Comression ratio? What are your timing specs? What fuel ( Octane rating and Company please ) do you use. Cast iron heads or aluminium? ( Aluminium worth about a 1 point increase in CR over Cast iron due to better heat absorbtion ).

If you're any where near 10.1 CR, detonation could be the problem.....

Get back to us. Late for work...gotta run.

 

im glad to see you Chickenman35

i have a different cam in the car now. it is 518/536lift and 280/288duration. i also have AFR aluminum 195 heads. my compression is 9.8:1. my timing is set at 12* btdc. i don't know what it is at full advance. i use 93octane usaully from a good gastation like mobile or something like that.

 

Time to update your signature . Well the change in cams and the Aluminium heads should certainly help eliminate detonation, although I still wouldn't rule it out. Do you really think it's backfiring through the carb or could it be detonation? Did this problem just start or has it always been like this?

If it really is backfiring through the carb...especially if it's a repeated stacatto like popping...you could have a broken or weak valve spring. That's a pretty healthy cam now. What brand is it? What are you using for valve springs?

A bad coil ( weak ) can also cause a repeated backfiring or popping through carb as well.

Really important to figure out if it is an actual backfire..or if it is detonation. Your description:

Sounds like you could be describing detonation to me. Hard to tell without hearing it myself.

If it is just one small backfire ( not repeated ) when you punch it, then you probably just need to go up a size on the Primary shooter. You've changed your camshaft from your previous setup ( and maybe your intake? Still running the Performer? ) and that may require more pump shot.

 

it might be detonation. im really not sure if it is a backfire. its really hard to explain. i have a band new msd coil that i had laying around, so i put that in and it did nothing. i messed around with different pump cams and got nowhere so i but the blue one back in. the problem just started a couple of weeks ago. the cam i have have now is a solid cam. do you think that one of the rockers might be loose. could that cause this problem. i just checked them about a month ago(before i started having this problem). the vavle springs are what ever AFR uses. they are also the ugraded springs from AFR that can handle up to 600lift. i don't think its a broken spring because i just got the heads in late febuary. i also have a victor jr intake manifold, not the performer. one thing i did not try was backing off the timing. i tried advancing it and that didn't do anything. thanks for the help, i really need it.

 

Well try backing off the timing and see if the noise goes away. But unfortunately you could have a worse problem...I hope not.

You mentioned you had a solid cam. If you are getting a repeated backfiring through the carb...this is a sign of an exhaust lobe going flat. Check the valve lash immedialtely. If you come across any that have mysteriously increased in lash.....this is a sure sign that a lobe is going flat. Usually an exhaust lobe will go first. The problem will get steadily worse, untill you can actually have flames shooting out of the carb. Hope this is not the problem, but a lash check should indicate whether you.

Big lift, large duration cams can go bad for any number of reasons. Proper breakin is critical. With a big cam like you have it is advisable to use " Breakin rockers " with a 1.2 to 1 ratio. Or if you have dual springs, to remove the inner springs during breakin. Lots of breakin lube must be used and you absolutely must check for coil bind.

Some cams are more critical than others in regards to the Valve Train setup and breakin procedures. Comp cams have very agressive ramps...and therefore if everything is not 100% perfect and if the proper procedures ( Breakin rockers or springs ) are not used they can give problems. More so than Crane or Isky in my experiences.

Best solution is to dump the flat tappet cam and go roller. It's really the only sure fire solution.

Hopefully you do not have this problem...but check that valve lash right away.

PS: Do try cranking down the timing...just in case it is Detonation.

 

ok its not detonation. i backed off the timing and it still did it. i had my dad listen to the car and he said it is definately backfiring through the carb. i will check the lash next weekend or if i get time during the week.

i always thought that you had to have the block machine to accept a roller cam. thats why i never looked into one.

 

Try advanceing the timing 12deg at idle is not enough for that cam.

try about 25deg at idle temorarily and crack the throttle.
If the problem goes away youwill need to recurve the distributor to allow the increased initial advance.
do not drive the car set like this as the overall timing will be way too advanced. What is the idle speed?
If it's just too low the car will cough when you crack the throttle reguardless of the setup.

Dirty spark plugs will do the same thing. Fresh ones always help throttle response.

 

Just a note on checking the rocker lash. Grab a dial indicator as well and check the lift on all valves.

When a lobe starts to go it is usually the lifter that fails first. Then the lash will open up. However, you can get considerable lobe wear and not have the lash change much. The nose of the cam will wear....but the base circle will not.

Checking the lift on all valves will definately show whether a cam lobe is going bad.

Roller lifters can be put into any block, whether or not it had an OEM roller cam or not. Just the parts change. Can be very expensive though, much more so than a flat tappet cam. The lifters aren't cheap. Neither are most solid roller cams ( Billet Steel ). Hydraulic roller cams are a bit cheaper as they usually don't use as much lift as a Solid Roller. Some " Street " grinds are now available for Solid Rollers. These are usually cheaper as they cam be made out of Iron Alloys like a Hyd roller.


However....if you want a big cam, roller is the only way to go IMHO. After a couple of teardowns and engine rebuilds due to large falt tappet cams going bye-bye ( trashing all the bearings and rings ) most people see the light and switch to a roller.

 

well i check the lash and all but one were fine. i pulled the intake manifold off and took that lifter out. that lifter was ****ed big time. it was worn really bad. i pulled the rest of the lifters and all the other ones looked like brand new. the one that was really bad was the number 3 exhaust lifter. tommorrow im going to pull the cam and check the bearings. i just wanted to let you guys know what the problem was. i also wanted to thanks you guys for the help. now i have to start looking at hydraulic roller cams.

thanks again

dan

 

Bummer..... I was afraid of that. Hope that too much damage has not been done.

Just a note on Roller cams. What sort of rev range do you plan on using. Hydraulic rollers are very RPM limited...especially with a big cam. The lifters are very heavy...really gets hard accelerating all the lifter mass up and down with big lift and duration. If you want to run much over 6,000 rpm, better to go with a Solid Roller IMHO.

" Street " Solid Roller cams are now being made by numerous manufactures. These usually have a bit less lift than a Race Solid Roller which makes the manufacturing a bit cheaper.

Now a really good way to save some money is to pick up a used Mechanical Roller Cam off of E-bay. Say one with a damaged lobe. You can usually pick one up for under $50 bucks. Because a race Solid Roller is made of billet steel....it can be reground to any profile.

Isky, Crane and Lunati are all fine brands IMHO. I have used a Comp Cams Hydraulic Roller ( XR288HR ) and was not impressed with the quality of finish. Poor machining on lobes causing some chatter marks on lobes. Core material seemed to be made of a relatively soft Iron alloy( relatively speaking )...which is probably why Comp Hyd Rollers are cheaper than Isky or Crane. Ya gets what ya pays for I guess....

 

my rev range will be around 6000 to 6500. thats what it is now. i want to go with hydraulic because i don't really feel like adjusting the lash, it get to be a pain in the ***. i think im going to call lunati and get their catalog. i can't seem to find a roller cam that is about the same size as the cam i have now. the ones i see are either really small or really big. there seems to be no middle.

 

well, i got the cam out. it alot worse than i thought. the bearings are scratched pretty bad and the one lobe is almost completely round. its really bad. now i have to pull the motor and get new bearings installed. i might ditch this motor and build a better one. im really not sure. kinda sucks because i don't have the money to build another motor so the car is going to sit until next summer. that really sucks.

 

Which bearings are scratched badly? Cam bearings or the Rod or Main bearings? If the rods and mains are OK and it's just the Cam bearings are just scratched...don't worry about it.

Cam bearings are pretty non critical in regards to condition. They can be scratched and nicked a fair amount before it becomes a problem. No reciprocal load...only rotational.

As long as the camshaft rotates freely and the bearings are not worn to the babbit, the cam bearings are servicable. Buff them with very fine Scotchbright...and they should be fine.

The Rods and the Mains on the other hand......well, they of course have to be in Primo shape.

 

well if i take the motor out to check the main and rod bearings i mine aswell just replace the cam bearings. i can only see the first bearing and its scratched but could probably be reused. but i can't see the onther ones

 

Note on Roller cams. Because of the higher acceleration rates of roller cams, a roller cam of a given duration at .050" equals a flat tappet cam of approx 10 degrees more duration at .050".

Once you are over .100" lift the acceleration rate of a roller cam far exceeds that of a flat ta[ppet. Those you have more " Valve lift area " when you plot the valve timing curve. Something to consider when choosing a Roller cam.

IMO, an Isky 201272/282 would be a good combo on your motor.

An Isky 201282/294 would be quite a bit more stout....but day to day drivablity would suffer.

A Crane 119831 or 119841 would also be a good bet.

Generally I would recommend the smaller of the cams if the car is driven on the street. You'll be much happier.

If it's mainly a strip car, then you can go with the larger profiles. I would definately recommend AFR's " Hydra-Rev kit with the larger cams.


Don't forget you must have good pushrods. Chome Moly with .080" thick walls minimum. ( Some Chrome Moly are only .060" or .065" thick...not good enough!! )

 

i think the crane 119841 cam would be pretty nice for my car. i pulled the motor today and ill check the bottom bearings tomorrow. hopefully everything will be ok.

 

well i checked out the bottom end and everything is fine. i miced the first cam bearing and i got 1.873(vertical) and 1.876(horizontal). the spec i have is 1.8682 to 1.8692 which means i am not within specs. so i guess i should get new cam bearings. i also wanted to know your opinion on forged internals. i don't have forged internals in the motor now. i know that they are stronger but do i need them for my application.

 

Cast cranks are just fine. Forged cranks are only necessary if you consistantly run over 7,000 rpm ( 350 CI ) for extended periods of time. By extended periods of time we're talking Road Racing. Blower applications and big shots of nitrous are an exception of course.

I don't know how hard you run the motor. RPM is the big killer, especially with the cubic inches you have. Below 6,000 I wouldn't worry too much. Above that and I'd start to worry about the rods. Nitrous and blower a whole different game.

For rods...it may be an advantage to spend some dollars there and get some good aftermarket rods. I've run fully prepped Chevy Pink and " X" beam rods for years with absolutely no problems. Run to 6,500 on a fairly regular basis at autocross ( 355CI motor not a 406!! ). Sometimes 7,000 RPM....but that's after I spent hours and hour's prepping the danged things...polishing, radiusing bolt pads, shot-peening, re-sizing big ends and adding ARP bolts. My labor was free...but I'd never do it again.

Again, Scat makes good products. Crower and Manley are also excellant and reasonably priced. I hesitate to recommend Eagle products as my Engine Builder has seen too many problems with them.

Remember that you'll have to rebalance everything if you change rods or any other rotating member.

 

i just have a stock 400 crank and rods. i rev the motor to around 6000 and sometimes 6500. i really don't have the money to go all forged right now, but i don't want to have any problems later on. i don't race the car that much. i bring the car to the track a couple of times a year. but i like to wind it out on the streets.

 

Crank should be fine at those RPM's . Rods the weak points. Try and hold RPM below 6,000 if you have the stock rods. Good bolts ( ARP ) an absolute must. Stock rods can work at 6,500...if you have good rods ( 5.7" long ....Wide beam " X" rods or Pink ) and if they have been prepped by an expert, de-beamed, polished , shot peened etc, etc etc. Good balance job essential as well.

If you have NOT done all of the above rod prep then you have two choices IMHO.

1: Buy some proper aftermarket rods. 5.7" preferred ( new pistons to match )

2: Keep revs below 6,000 as much as possible. A 406 is not a Rev happy engine...I really don't think you will see any significant gain in going up tp 6,500. There's a hell of a lot more reciprocating mass being flung about with a 406 than a 350. And if you're using the stock 400 rods (400 rods are 5.5" long...350 and smaller engines are 5.7" long ) that makes the situation even worse....high rod angularity which results in very high thrust loads on the piston face. With the short 5.5" 400 rod, all you achieve with running high RPM is wearing the bore and pistons faster.

 

im pretty sure that i have 5.7 rods. i think im going to save my money and bring the motor to a good machine shop and tell them what i want. i'll have them do all the work. also if i get aftermarket rods and pistons, should i get full floating pistons or the pressed type. should get forged pistons because i have hyperutectic pistons right now.

 

Full floating does not really matter in a street motor. Only advantage with full floating for a race motor is that it allows easy replacement of a damaged piston.

What type of Hyperutectic pistons do you have? The Keith Black ( Mfg'd by Silverlite) " Signature " series are pretty good, but my engine builder has seen lots of the " Standard" variety literally disintergrate under heavy use.

If you plan on using Nitrous or forced induction later on...then Forged pistons are an absolute must. Be concious of weight with Forged pistons. Some are absolute bricks, such as some of the cheaper TRW units. They are strong...but heavy. Now TRW does make a lighter " Competition " version...but it is double the price. Ross makes some very light pistons....fairly reasonable from what I have heard. Your engine builder should be able to help out with piston choice.

BTW, ran some dyno simulations on you engine using Desktop dyno. Don't worry about the figures...but the RPM trends were very interesting.

I assumed small tube headers ( anything under 1.75" calculated as small tube ) running through mufflers and a 112 degree LCA on your cams.

Summit cam:
Peak power at 5,500 RPM = 434HP
6,000 = 418
6,500 = 388

Crane 119841 cam:

Peak power at 5,000 rpm = 447HP
5,500 = 441
6,000 = 417
6,500 = 374

Note the RPM trends and power drop off. Not much point in going to 6,500 rpm is there. Torque figures are not shown. Crane cam made more torque with nearly 500ft lbs!! Will re-run program later this evening.

Couple of notes. I'll redo the figures on the Crane cam...I think it's on a 110 degree LCA but I have to calculate the specs as it's not listed. Can you supply LCA for the Summit cam?

The Performer Manifold is really hurting your performance. It simply cannot provide the airflow that those heads and cubic inches require. It is really a low rpm range, torque manifold ( you have no shortage of torque...the AFR 195's are being choked by this manifold IMHO )

A Performer RPM would be a much better choice...it is miles ahead of the Performer im every respect. Hell...with those cubes and heads you could easily run a Victor JR. on the street. Would really wake up the motor. That would be my choice.

 

im going to give all the specs that i have for my motor.

400sbc
bore: 4.165 (.040 over)
stroke: 3.750
total vol: 408.7ci

compression ratio: 9.85:1

heads
AFR 195cc aluminum heads
intake valve: 2.020
exhaust valve: 1.600
74cc combustion space

induction
holley 750dp
victor jr intake

exhaust
small tube headers with mufflers

cam
i plan on using the cam that you reccomended(119841)


now you know all my specs. i think its time to update my sig. the pistons that i have now are speed pro coated hypereutectic. i got the kit that summit sells on pg 115 of their new catalog. also what is the highest compresion i can run on 93 octane. thanks for your help. i really appreciate it alot.