383/496 stroker bogging on highway

[Matt DeCicco]

Ok so here's the deal,
I have a newly built 496 stroker. Has aluminium head, hydraulic roller, mp performance ignition. Holley blue pump set to 6.5psi, holley 750 mechanical. The car is a 4 speed. Timing set to about 13°, about 8 HG vac, floats adjusted, acc pump adjusted, 6.5 PV (have smaller coming).

The car goes like hell 1,2,3 shift right up to 5,500. Once I hit 4 it starts to bog/ cut out like it's running out of fuel. If I cruise up to 4th on highway, as soon as I get over 3,000 rpm it does it again. When it happens I have to put the clutch in and feather it to get it back to normal, it takes a few seconds and backfires through exhaust a little bit, sometimes it stalls.

I had the car running decent and this issue was not present, but I noticed the throttle cable wasn't opening fully so I adjusted it and got a lot more power but now have this issue! I'm not sure if it's timing related, maybe need to adjust vac advance or spring in distributor? It's strange that it's only in 4th over 3k. Thanks for the help!

 

[Matt DeCicco]

Forgot to mention 11.5:1 compression

 

[Xp29h]

Is it a regular throttle cable set up or Lokar or similar type? Interesting that it is doing it after you fixing cable.

 

[Matt DeCicco]

Lokar type. It almost makes sense to me because before, I wasn't getting full throttle so I had less fuel requirements. Now it needs more fuel fully opened. But I thought highway in top gear I would be using the least amount of fuel.

 

[texas69bee]

At 3000 rpm you have max vac but not enough for the power valve you have in it. It may clear up after the other valve is installed.

 

[Xp29h]

I remember on my Lokar setup that the back grommet that goes into the throttle bracket got dislodged after I adjusted it and wouldn't allow it to open up and transmission wouldn't shift right either. But that was with an auto trans not 4 speed. You've probably already done it but I'd double check the connections up through just in case.. That would be simple check.. Probably not the fix but worth a check..

Good luck

 

[RT/10]

It sounds to me like your running out of fuel supply. Does it do it even when the tank is full? If it does it then also you can rule out uncovering the fuel pickup in the tank. If it's ok with a full tank your uncovering the pickup and the only solution is always keep a full tank or put in a sump. What size fuel line are you running? With a 496 (even a mild one) your fuel requirements are going to be pretty high. If your not running a 3/8" line (or bigger) you need to be. Also if the fuel line has any sharp bends anywhere that'll drastically lower the fuel output. What about the fuel filter(s)? Are they of good quality and not restricting the flow? You need a coarser filter on the suction side of the pump (50 microns or larger) and on the output side you can run a finer one (10 micron) to protect your carb. Where is your fuel pump located? If it's not real close to the tank and no higher than the pickup that could also be the problem because electric pumps don't have a lot of vacuum to pull fuel a long distance or uphill. I had the same issue as you with my 68 Cuda when I installed a 408 in it. Had a Carter high output mechanical pump on the old 360 and things were fine. Wouldn't do the job for the 408. Tried a new Edelbrock and Holley mechanical pump with the same results. Solved it by putting a Edelbrock HP quiet flow elect pump back by the tank.

 

[oldbee]

Ditto what rt/10 says, you got fuel press but maybe not enuf volumn.

 

[Matt DeCicco]

Hmm good points. I have 3/8 back to the tank, maybe 4-90° bends and a 45. The pump is mounted low on the subframe connector under drivers floor. I have tried it with a full tank. The filter is a canister type, Wix 6 micron filter. Maybe the filter is too fine? I'm going to order a different filter tonight and the new PV should be in tomorrow so I'll give that a shot. Thanks for the help!

 

[Matt DeCicco]

Ok here is my update. I put in a holley black pump, fuel pressure is good while driving, 950 proform carb, nothing helped. I set initial timing to 13° with vac advance and put one stiffer spring in the distributor and it is much better. I can now go on the highway ok, but it feels a little sluggish when I first get on it on the street. I think it needs a little more initial timing but the idle is at 1,100 and will not adjust lower when I advance it. Thoughts??

 

[RT/10]

What is the total timing and when is it all in by? When you say the idle won't adjust lower I assume you mean you have the idle speed screw adjusted all the way out?? If so make sure the throttle cable isn't limiting it from going all the way back. If the throttle cable isn't offending it sounds like you need to back off the idle speed adjustment on the secondary side a bunch. BTW: have you pulled the plugs and had a look at them?

 

[Matt DeCicco]

Ok, so I replaced one spring in the distributor to a stiffer one and this seems to have eliminated my problem on the highway. I also have an air fuel guage connected. My vacuum is low and the needle fluctuates at idle between 6-7 HG, also when I increase rpm the vacuum increases as well.. Thought that was strange. Maybe a valve needs to be adjusted? When I hit the throttle quick and let off I do hear a faint click.

 

[oldbee]

Vacuum leak? Did u check at least one plug to see what it looked like?

 

[Matt DeCicco]

The only vacuum lines are vac advance and pcv, I sprayed around intake to check and all was good. I did a compression test today 190 on every cylinder, initial timing 17° and about 40 all in by 3,000. Plugs look good and air fuel ratio is good.
The car runs good, I'm just thrown off by the vacuum going up with rpm, does it need to have a load on it to drop? The rear main seal seems to leak slightly on high rpms so I'm wondering if low vacuum is causing to much crankcase pressure.

 

[RT/10]

It's normal for vacuum to go up with RPM at low speeds. At low throttle opening it will keep going up till you get to a point where the throttle blades are quite a ways open (usually over 1/2 way) then it'll start dropping. You said you put an A/F gauge on it. Is it a wideband or narrow band? What A/F readings were you seeing? Vacuum has very little to do with crankcase pressure. I run pan EVAC system so at idle mine has almost nothing pulling on the crankcase yet I drive it over 2000 miles a year and have absolutely no blowby. Your rear main leak is just that, a leaking rear main. Even sometimes new motors will have a small leak and crankcase pressure will not affect a rear main leak. It can cause a leak at the pan at the rear but not the rear main seal itself

 

[Matt DeCicco]

A/f is wideband. Cruising at 3k it's around 13. When I get on it, it leans out for a sec but stays steady around 14.5-15. if I'm in a high gear around 2,500-3k, say 3rd, and get on it without downshifting, it leans out and will backfire through carb. I think maybe going up a jet on the secondaries might help? I think I'll change the rear main to rule that out, maybe the rope one from Mancini. The leak seems to have gotten a little better, but still when I get on it hard or drive on the highway I'll have drops on the inspection cover and passenger collector.

 

[451Mopar]

Sounds like it is lean, but is it jetting or fuel supply? How is the tank vented?

 

[RT/10]

A/f is wideband. Cruising at 3k it's around 13. When I get on it, it leans out for a sec but stays steady around 14.5-15. if I'm in a high gear around 2,500-3k, say 3rd, and get on it without downshifting, it leans out and will backfire through carb. I think maybe going up a jet on the secondaries might help? I think I'll change the rear main to rule that out, maybe the rope one from Mancini. The leak seems to have gotten a little better, but still when I get on it hard or drive on the highway I'll have drops on the inspection cover and passenger collector.

13 at cruise is a little rich IMO. Unless your cam has a bunch of overlap you don't usually need to cruise that rich. And 14.5-15 is way lean when your getting on it. Depending on whether or not your running gas with ethanol your WOT A/F should be in the neighborhood of 12-12.5 to 1. Also sounds like you need more accelerator pump shot and/or duration. Quick revving/large engines like a lot of fuel when you hammer them. What size squirters are in it and what color pump cams?

 

[Matt DeCicco]

13 at cruise is a little rich IMO. Unless your cam has a bunch of overlap you don't usually need to cruise that rich. And 14.5-15 is way lean when your getting on it. Depending on whether or not your running gas with ethanol your WOT A/F should be in the neighborhood of 12-12.5 to 1. Also sounds like you need more accelerator pump shot and/or duration. Quick revving/large engines like a lot of fuel when you hammer them. What size squirters are in it and what color pump cams?

 

[Matt DeCicco]

This is great info, I appreciate the help! I agree with the accelerator pump duration.. I have 31 squirters and I went up a size to see if it would help, but didn't do much. It leans out when I hit the throttle for a second. The cams are whatever came on the carb, maybe I'll pick up a set to play with. Do you think the 750 could be too small? I put a proform center on it to help with air flow. I'm not sure how to get wot to get into 12's for a/f.. my rear jets are maxed out from the kit I have, maybe go up a size in front?