So I just got this truck running last night, some of you may have seen the thread on it (https://www.theturboforums.com/smf/index.php?topic=73635.0). It's been a handful from start to finish, but it turned out pretty good I think. After the initial test drive I had some problems, I've already PM'd Brent and briefly filled him in on the details.. So I though I'd give you guys the rundown too and see if there is any different input.

Combo includes:
355 cu. in., AFR's, small cam, Torker intake, HEI locked at 23*, complete custom turbo kit w/ Turbonetics T70, Air/Air, CSU blowthrough 750, Complete A1000 fuel system, 13202 regulator, -10feed and return, sumped tank, CSU bonnet, etc. (I think that covers all the bases)

The problem: The truck drives perfect under light throttle and NO BOOST. Once it gets the slightest hint of boost, it stumbles and pops through the carb. If you get on it hard it does the same thing but the engine revs freely - just with a heavy miss throughout the RPM's.

I don't have a wideband on it just yet, but it sounds like it's severe spark blowout. The hard part is what came first - the chicken or the egg (the pop through the carb or the missfire). I couldn't get the right plugs (NGK's) so I had to get the Autolite 3924's which have a long extended porcelan on them.

Ideas?

 

What is the plug gap, if the plug wires are old, and there is boost they will not fire the plug, it will start jumping other places.

 

Plugs are .035 right now. Wires are new.

 

How much boost, when you say any boost. Where is the boost pressure ref at.

 

I won't have the guage in it till tonight, but when I say "any" boost I mean, as soon as you can feel it - it stumbles. I'd say 1 or 2 pounds at the most is when it stumble and pops through the carb. Like I mentioned to Brent - I'm not sure that the missfire isn't causing this as well. When you just nail the gas it pops through the carb once or twice and then just revs semi-freely with a bad erratic miss. The reference for the FPR is in the bonnet if that is what you mean.

 

I don't think the plugs would cause that popping and stuff with just a hint of boost but as mentioned before, the jetting or AFR would help to see what it is doing at that moment. You can also take that timing up to 38* and see what it does then but don't stay in it unless there's a retard or timing computer to take timing back out.

Definately keep us updated and boy do you do great work! I guess the red hatch may see blow-thru one day hugh?

 

Brent, Thanks for the props man! I have a friend coming over tonight to let me borrow his LM1 so I can test it out for sure. It doesn't have any kind of timing control though, so I can't advance it very much. Like you said, the AFR will tell the whole story though.

I doubt the red hatch will see the likes of a B/T on it, I have WAY too much money tied up in electronics to switch over

 

Get some gauges hooked up.. Boost, FP, and a WB02 You can try tuning it by plugs but with the ease of what the electronics provide I would use them. A lot of unknown variables here. I doubt you are blowing out spark but you could gap down to .028-.030 and try it as long as you know other things like FP and O2 are doing.

Walt

 

yeah, I agree, it really needs the guages in it - it was one of the hurry up and we can go drive it kinda things ya know. I didn't put the guages in yet because he spent the big money on the electric cobalt guages. They require all kinds of wiring and stuff. >

Something else I noticed and just rememberd, the accelerator pump seemed pretty tight too. It wasn't contacting the arm squarely when I put the carb on so I had to bend it in towards the float a little bit. That may have tightened it up even more.

I have a few things to check out, float level, install guages, install new plugs etc.

 

I have always been taught that backfire thru the carb is timing. Period

 

A lean spike can do that as well though, and of course a really rich one can do it too.. but a rich pop through the carb is pretty easy to tell..

 

I agree

But a pop to me is significantly different then a backfire.

I am not qualified enough to diagnose a carb problem over the web....

 

LOL, I've verified the timing though... so it's narrowed down some, but not much.

I'll report back tonight after I put the guages in and get the wideband on it.

 

what kind of cam specs? it sounds to me like timing issues, or cam timing.

 

is this lean spike in the primaries, kinda like subtly on the throttle? and it seems like it just goes dead and then booww or kinda just has no throttle response like it wants to stall in the primaries??? If so thats the same prob i have....... and i have no clue why so.... i can relate.

 

It has NOTHING to do with the cam, since the motor ran fine before the turbo. It can't be timing related either since it's set and locked at 24*. The truck starts, idles, and cruises perfect.

It seems like 2 seperate problems. I think the missfire is the plugs... The other part, the pop through the carb, is a lean spike because it ONLY happens on the first stab of the gas and ONLY when it starts to create boost. I should get it ironed out tonight. I hope.

 

Personally I think the plug gap is way too wide. On my SBC I run my plugs at .025" gap. I run NGK 10s. At .035" gap it will be easy to blow out the spark with almost any boost. Even on my nitrous setup I had them gapped tighter then that. I would start by tightening up the plug gap and doing a double check of your cap, rotor, and wires. Also make sure your rotor is phased correctly because if you have to jump a large gap in the cap it will further reduce spark energy. Also...what coil and ignition box are you using? Are you running a crank trigger?

With the pop thru the carb....sounds like you need more accelerator pump cam and/or squirter. You could be having a major lean area under transition from idle to part/full throtte and that is usually accelerator pump related.

 

It's an HEI, No crank trigger, no phasing, no ignition box, timing locked out at 23-24*. I am getting ready to go outside and put some new plugs it with a reduced gap, also with a non-protruding porcelan tip.... I'll be back in a little bit.

 

Ok, here's the scoop. I changed the plugs and bummed a LM1. I still didn't get the guages in it yet.

The thing idles perfect at 14:1; cruises perfect at 14:1, once it gets any slight load on it the A/F goes through the roof. If you stab it, it temporarily stabilizes, lean pops through the carb and it's time to let off.

The plugs made a helluva difference for sure, but didn't totally remedy the problem of course. I also found the #6 plug wire hanging off of the plug, but I don't know if I drove it the first time like that or not.

So, tomorrow, the guages go on and what appears to be a jet change as well. The only reason it wouldn't need to be richened up is if it were not rising the fuel pressure 1:1.

Dave - the reference is coming from the bonnet @ about the 9:00 position.

 

Yeah, I pulled the fitting out of the regulator and sealed it up - or at least I am pretty sure I did. It may have been one of those moments when I thought about doing something and never did it. That will be the 2nd thing I do tonight, the first thing is the guages.