overcammed 350

[85_prairieboy]

Last year I put together a 350 bored .060 over together. Originally I was using ported camaro heads with 76 cc chambers and last year I switched to AFR 195 Eliminators with 65 cc chambers. I used a bigger cam to bleed off some of the extra compression I thought I was going to have. By the way I am using Speedpro forged pistons with 2.4cc dome and the block is zero decked. .039 head gasket. I used a Comp Magnum solid lifter cam installed straight up . http://compcams.com/Cam_Specs/CamDet...?csid=221&sb=0

Now the engine has no low end torque and if you open the throttle too much before it revs up it pops back through the carb (750 dp). Today I did a compression test and found 165-170psi. Should I be switching to a smaller cam or am I missing something? Feel free to educate me. I want to do my homework this time before i buy a new cam. If you need more info I can get it for you.

Steve

[djracer]

tell us more about the combo. What tranny? What intake? the more info the better.

[tubbed 68]

u may have a vacume leak around the intake runners check there first. did you cross a wire.

[hotrod 80]

Cam card shows 2500-6500 rpm . If you don't have enough converter it's gonna be a slug down low . I agree need more combo info .

[85_prairieboy]

I am currently using a th350 with a 3000 B&M Holeshot, 3.42 gears, a Weiand Stealth intake, 36 degrees total timing, HEI with MSD coil, Mallory 6AL box, full length headers dual 3" exhaust with h pipe.

The motor is in a roughly 3900# truck.

[shortbed70]

DP is the problem I have had them do that before #1 I think its too much carb and #2 I had a warped base plate holley750 that made my car pop also 78 monte. Let me guess you stand on it it pops then get off then back on it right away it wakes up. Try swapping to a vacume secondary carb. Also what lift on the cam? Don't forget that if your motor doesn't make alot of torque your convertor might not stall where advertised.

[djracer]

the exhaust sounds a little big. What size hedders?

[Wild83C10]

I don't think the pop is from the cam. I do think you have too much duration for that motor.
I had a similar setup before the 383. But with a victor jr, more lift and a little less duration. it wasn't the best off the line, but it pulled up top.
Those heads flow real well all the way up to .600, more lift and less duration would probably help. Talk to a cam company. Cammotion is pretty good, don't have to wait on hold for hours like comp.

[85_prairieboy]

Shortbed 70 You could be right about the carb. I spent a lot of time tuning that thing. It sounds like a lean backfire. I got the best results by switching to the biggest squirters the 30cc pumps would allow(i forget what size) I was going to swap in the 50 cc pumps but I don't think this carb was meant for heavier vehicles. BTW you are bang on with the drivability description. I think I am going to get a 770 Street Avenger. My brother has one on his Camaro so I am going to try his out.

djracer I have 1 5/8 headers. Not sure of the brand, they were on the truck when I got it. I put the 3" exhaust on thinking bigger is better but since then people have told me the same as you. Is it the exhaust scavenging that is affected when you use an exhaust pipe too large? What size should I be using?

Wild 83c10 I have been reading a bit of David Vizard's stuff and he said that low cranking pressure can be an indication of too big a cam assuming the rest of the engine is sealing well(which it is). He recommended doing a compression test and used the benchmark of 200 psi minimum for a performance street engine. I found 165-170 psi which could mean the pistons are already a long way up on the compression stroke before the intake valve closes. No wonder this thing is such a dog off the line. It really pulls hard once it gets to 3500-4000 and then is strong right to the limiter at 6000. makes for impressive rolling burnouts but sucks trying to leave the line quick.

Wow! I probably screwed up everywhere I could have with this combo. Its depressing but also exciting to think of the potential it has if I get everything sorted out

[djracer]

I would think 2.5 exhaust would be max till you get over 400 inches. Then I still wouldn't go to 3 inch unless it was a stout 400.

When you did your compression check did you have the throttle open at all? I have seen people have funky readings with the throttle closed.

[Marv D]

I also see some room for improvement in the combination, but..... I'll bet a dollar to a donut this is nothing but a tunning error. Take it to someone that has a chassis dyno and let them tune it with the proper equipment and I bet you see a 100% improvement in drivability and performance. I used to think dynoing a motor was to see how much power it could make, NO, you will find every little hiccup inthe tune on the dyno. Most places charge about $500 a day for a engine dyno, and $250 for a chassis dyno tune. Trust me,, it's worth EVERY penny.

my 2cents and maybe worth 1 of them

[Wild83C10]

Did you dial in the cam when you installed it or just installed it straight up, advanced?

I agree with Marv, unless the cam is installed way off (advanced) I'm sure its a tuning issue.

What size jets are you running? What size power valve? What about the squirters?

[85_prairieboy]

Thanks for all the help so far guys
I did my compression test with the throttle wide open and the battery fully charged. The engine was warmed up too. My previous combo with a smaller cam and iron heads tested at 185 psi so the lower numbers now stood out to me. This engine might have 1000 miles on it. I installed the cam straight up and did not degree it. As far as my carb tuning I am going by memory. I think I ended up with 68 primary 74 secondary, 4.5 or 5.0 power valve and #37 squirters. Since I maxed out my squirter size I was going to swap in the 50 cc pumps but I wasn't sure if I was just putting a bandaid on a bigger problem. I now want to use a vacuum secondary carb unless you guys think I can work out the bugs with a little more tuning. I have been told the dp carbs are not really compatible with heavier vehicles. If the backfire is just a tuning issue it probably is a lean backfire which is what I thought initially when trying to tune the carb. It was really bad when I first put the carb on but I was more concerned with the overly rich mixture at cruising speed. I started with using smaller jets until I found the ones that would give me the best color on my plugs. Then I figured out which power valve based on Holley's procedure. Went from a 7.0 to a 4.5 or 5.0(can't remember). After that I turned to the lean backfire. It seemed to improve with the bigger squirter size but the backfire I am still getting must be happening once the engine uses up the squirt of fuel. I would then need to get bigger pumps but by then I was starting to question if I should have used this carb. Do you guys use mechanical secondary carbs on your race setups?

[slednD1]

I don't know your mechanical ablilties or who's help you are getting but you may want to start with the mechanical basic's.first verify cam anlge and timming pointer angle especially if your new crank gear had 3 possitions,i have even seen them installed wrong by people i consider decent mechanics.next re-check your valve lash,if you need help I can.then double check int. & carb sealing.set initial ign. timming to 10-15 deg. w/vac.adv.disconnected and vac. port plugged.next check port vac."below throttle blade"at IDLE 900-1000rpm if you get 15 in.use 7.5 power valves front and rear!use about an 80 jet front and rear,dual 50cc pumps w/pink cams pos.2 making sure you have .015" or more pump travel left.secondary throttle stop 1/4 turn from closed,#35 pump discharge nozzles have more pressure than #37's/smaller main emulsion bleeds will change the booster delay off idle and will create power before you hit your stall ?3000?

[slednD1]

also: loading up at idle will be a problem and may cause your current prblem.I looked at your cam specs and that thing should run great when tuned.because of your 106 lobe sep.you are only gonna get about 10" vacuum,this results in having to set primary throttle blade angle to far from stop just to get it to idle,prblem is this shows too much transfer slot and now runs rich at idle.to get throttle blade back to 1/2to1 turn off closed you must drill small "less than .125" holes in each blade on the transfer slot side.idle mixture screws should be from 1-1.5 turns out,less than a turn out is not enough fuel for your motor to idle even if you think its rich,you just need more air.and i hope you are not running a pcv on one side and a breather on the other.that is a vacuum leak.if pcv present you should seal the other cover.pcv's contaminate intake charge+hot air in your runners bad idea. good luck

[85_prairieboy]

Thanks slednD1! I am actually a Journeyman tech but I am used to working on modern stuff. This is my first experience with Holley carb tuning and I have learned a lot along the way. I think everyone is in agreement that my major problem is a tuning issue. In a few days it is supposed to warm up again so I might play with my carb a bit more. I haven't been able to get my truck to idle without me sitting in to keep it running. I was close to drilling holes in the throttle plates because the transfer slots were exposed but several people stopped me and said I was ruining a good carb. By the way, yes I am using a pcv on one side and a breather on the other. I didn't realize this was going to cause a problem

[slednD1]

well get rid of the pcv and put another breather there for now and then re-ckeck idle vacuum to decide what power valve to use "1/2 of measured vacuum".as for the holes,if you cant afford a new cfm carb then holes are the way to go.as a tech yourself you know that cfm ratings for a 302 ford 5.0 throttle body go above 1200cfm,and that cfm ratings are how much air not fuel they flow.so heres some food for thouht,the air gap around your 750-850 throttle blades has a certain amount of air bleed at closed throttle.so at idle an 1150-1250 cfm carb will flow more air,not more fuel which will lean your idle and create higher RPM potential.let me know how you like these little trick's and secret's

[djracer]

Are you running power valves front and rear? If not I don't think you need the jets squared. Normally you run about ten steps larger without a power valve. I would say 72s and 82s if you don't have a power valve in the rear.

[slednD1]

yep i run power valves in rear too.think of each barrel as its own carb and keep adjusting the four of them together until you reach your goal or desired tune.then when you get close enough to perfect set your ign. mechanical advance to full at 1000rpm higher than your stall.3000rpm stall go full advance by 4000rpm

[85_prairieboy]

Do you use a different metering block in the rear or do you just pull out the plug and thread in the proper power valve? I have heard of blocking off both but not using two power valves. It makes sense though that the fuel would be metered based on the engine load in the secondaries as well. I might just stick to figuring out my lower rpm tuning before I start playing with that though

[prostreet1967]

i just ran into same problem a buddy out a 355 together with 562 int 550 exhaust installed straight up he told me it was popping through carb i checked exhaust temp on each cylinder and found cylinder #3 was only 150* while the rest where close to 600* did compression test it was good pulled valve cover and found he forgot to set cylinder #3 exhaust valve.

[slednD1]

same metering block,yep just put the right valve in it too.and your right on with your makes sense statement,that is exactly your goal.you do have four individual carbs to deal with.you seem pretty knowlegeable and crafty,you will figure it out.that lean pop off idle gets a lot of guys second guessing themselves,usually because they think its rich when they havent fixed the idle circuit yet,so they try to lean it out by changing jets,which makes the problem worse.drill those 3/32-1/8" holes and get those pumps in and start feed'n those pony's,if you ever want to talk shop i will pm my phone # to ya!

[americanmusc1e]

too steep gears for that cam and that weght, switch to a 4.10 or 4.56 or a 3.73 for gas mileage, Whoever said switch to a vacumn secondary carb was also on the right track and I also agree with the dyno tuning

the burnout problem is probably more gears than tuning though

[team39763]

Did you ever sort this thing out? I was having very similar problems, but my cam is much smaller than yours. I installed 50cc pump, huge squirters, changed cams...all that stuff and it SEEMED to run a bit better. It turns out that my intake gaskets got blown out, but I couldn't tell without removing the intake. I remember I would spray the motor down with carb cleaner, WD40, soapy water...never found the leak. A guy on a different site suggested I check the intake gaskets and sure enough that's what it was. I put new gaskets and my truck ran better than ever. It starts everytime and I didn't have anymore popping back through the carb. Good luck sorting it out if you haven't already.

[bigt76_98]

a 350 on pump gas can have more cranking psi than a 10:1 396.different variables change this just be worried if there is a big flux in the psi from cylinder to cylinder. fact