Stumble/backfire under load after installing Howell TBI

[jbenguerel]

I just installed the CA legal kit from Howell and it runs great EXCEPT for when it hits about 3k rpm. It does it whether in gear or sitting in the driveway in neutral. I changed the plugs, double checked my vacuum line connections, changed to a larger fuel return line (although I did this before noticing the problem), and checked the timing. I have an MSD pro billet distributer and I've tried to eliminate as many possibilities as it could be. It didn't do it before the swap so I know it isn't an internal engine issue. Has anyone else experienced this problem? My brother in law was thinking that it may be the mechanical advance but I don't have the slightest idea...

[AlsChopShop]

have you checked your fuel pressure?

Al

[jbenguerel]

not yet because there was such steady pressure coming out of the return that I didin't think that it was an issue. that is my next step...was just trying to avoid buying more parts/tools considering how much i've spent over the last week. what should the fuel pressure be?

[chef]

If the fuel pressure is fine, I'll bet it's running lean. Had a hellova time with my Howell. Stock motor?

[FSJ Guy]

GM spec for fuel pressure was around 13 psi or so, IIRC.

If you can, remove the air cleaner lid. Shine a flashlight onto the injectors. Have a friend rev the engine up to 3K rpm and watch the spray pattern. See if there is any noticeable difference.

A fuel pressure gauge is around $30 or so.

[jbenguerel]

yeah stock engine. that is an obvious way to check to see if its getting fuel and i feel dumb for not thinking about it. lol. i checked for vacuum leaks again last night and even checked to make sure that the fuel lines weren't collapsing after reading a post from someone who had the same issue and posted a pic of the fuel line collapsing under load. my brother in law mentioned it possibly being the mechanical advance advancing too much under load...the only reason i'd question that is that we tried retarding the timing when we were checking it to see if it eliminated the problem and there was no change in the hesitation regardless of how we set the timing.

[PlasticBoob]

Could be tach line noise, is there a way to see your tach pulses? Should be smooth and even with no dropouts or misses. Make sure your tach wire is routed away from spark plug wires and the coil.

[jbenguerel]

BINGO! The spray has very brief interuptions only at high rpms. The reason that I changed the return fuel line was that it was running too rich. So after noticing the interuptions I partly clamped the return line down and it made it better but didn't fix the problem (the result I expected). So this tells me that it must be the pump, the incoming fuel line, or possibly an electrical interuption (unlikely in my mind but I know some of you are a lot more knowledgeable than me). I'm thinking of running a flex line all the way back to the tank rather than using the metal lines as recommended by the instruction manual and seeing if that solves the problem rather than spending time and money on the fuel pump that worked when it was in the '77.

[PlasticBoob]

For each tach pulse (cylinder) the computer sees, it cycles the injectors. Noise can creep in at high RPM and load and cause tach pulses to get dropped; subsequently the computer will not squirt the injectors during that time. It could be what's going on, though it's unlikely that's your problem because I rarely see anyone have noise issues with Howell. It's a good place to start, however, because the entire system depends on a good tach signal. Better to rule it out now.

[jbenguerel]

huh...good to know. that would be a possible expanation. i've been under it all morning...tried changing fuel lines and after reconnecting i can't get it to spray any fuel now. i've even switched back to old fuel line and nothing. it is causing me to believe that the problem lies with the fuel pump...i've turned and turned the engine, letting it cycle mutliple times before trying to fire and it just isn't happening. can i test the pressure without fuel getting to the tb?

[FSJ Guy]

rather than spending time and money on the fuel pump that worked when it was in the '77.

You're using the TBI spec'd pump, right?? You can't use an electric pump designed for a carb. The fuel pressure is too low on those. That would explain poor running above 3K RPM.

You need an E2000 pump or something similar.

[jbenguerel]

no. i'm using the pump that came with the kit. i opened it up and there was $h!t inisde of it. cleaned it out as best as possible by tapping it out and then with the air hose but so far nothing new. still giving me a headache

[jbenguerel]

well i just ordered the new fuel pump. keeping my fingers crossed that all the issues are resolved after i get it in. we shall see

[PlasticBoob]

Interesting.... are you running a prefilter before the pump? Don't rely on just the sock in the tank, use one of these: http://www.summitracing.com/parts/HLY-562-3/

[Michael]

Is it backfiring through the throttle body or the exhaust? If you pinch the return line you just added fuel to the injectors. What is your fuel pressure? I do not run a Howell but have run a Holley tbi for 7 years, they have an adjustment on top of the inlet of the injectors, it was set at 8psi from Holley, a buddy of mine has a Howell and the throttle body looks the same....I bet you need to adjust the fuel pressure, here at almost sea level if I run below 11 psi it backfires through the carb at 3000 rpms or heavy load....I run 13psi. But obviously it is a little different system. Good luck