Carb Problems

[C4CorvetteGuy]

ok, here's the issue, understanding that this is my first true muscle car, i need some help, but the good news is that i'm not past admitting it. I have an '85 Corvette C4. i love this car, however, the previous owner did something a little strange... he replaced the tuned port injection with a 650 Holley DP carb (not sure on the model number, will post later this week) in any case, it ran great, it had a few kinks in it (i bought the thing for 7 grand) but overall a sweet deal, a 383 stroker, high rise intake, gapped rings (i assume he planned on nitrous, although not entirely sure) and he did the entire exhaust, from the block back... it's exhaling through hooker headers to a 3.5" H-pipe Flowmaster exhaust... so the problem is in the carb as you might have guessed. in short here are the symptoms:

Sluggish Acceleration (i can barely get going from a stop sign)
carb will pump enough gas to get to 2000 RPM, after that it cuts out, all i hear is a hissing sound, and my engine dies, if i try to just pump the gas, same hissing responds only after 2000 RPM and dies after i take my foot out of the gas.

i don't know if this is due to med-grade fuel, or if i just need to rebuild my carb, but i would appreciate any input in this area, as i do not want to waste a lot of money to replace what is all in all a $250 part or a $50 rebuild, thanks in advance, and i'll catch you later.

[MrPbody]

C4,

Welcome aboard.

A couple of things. First, Corvette is a "sports car" (2-seat, primary design for road handling, straight-line performance is secondary), not a muscle car (intermediate body, large displacement engine, straight-line performance is primary). Second, a $50 rebuild should be avoided at all "cost"... A quality rebuild for a Holley costs at least 2 hours labor and a $40 kit. Under no circumstances should a kit that doesn't say "Holley" (not "FOR Holley") be used.

A 650 is a bit small for a 383. We prefer to use a 750. If buying a new one is an option, check out AED (Advanced Engine Design) in Richmond, Virginia. They offer the best "out of the box" Holleys we've ever seen, much less used. Their "750HO" is ideal for your application.

Be sure the ignition system is correct, too. Over-riding the computer system can become a "bag of worms" in a car of that era.

If you can provide more specifics on the symptoms, I may be able to narrow it down for you. Baisc tests are in order, such as fuel pressure/volume, ignition timing, etc.

Jim

[Schrade]

You should get the TPI setup back on the motor. They can be had cheap from FParts ebay.

Is the original fuel pump still in the tank? I think you HAVE TO know that...

[C4CorvetteGuy]

wow, should have thought of that... about the fuel pump, an aftermarket was installed on it... from holley, also, let me go ahead and clarify, when i said $50 rebuild... i meant me buying the kit (apparently i rounded up too high?) and rebuilding it myself... aside from my term replacement (sports car and muscle car) i wouldn't call myself a retard, and i'm pretty mechanically inclined, i'm just assuming that some of you guys out there are a little more experienced than me, a lowly 19 year old. in any case, i agree, the 650 felt small (it was running fine not but a month ago) but on the other hand, when he put in the high rise intake, the inlet of the tiny 650 fit right to the hood... not the most ideal place to put an air filter, so he cut a hole in the hood, and put an intake right there... it's rather small, and i'm not pleased, i'm not looking for peak performance (as in with all the money in the world, i have other plans i will elaborate on at the end), i just want the damn thing to run, i'm about to move in 3 months, and i don't have a lot of time, or money, i would just like some advice on areas to look at that i haven't already... a buddy of mine told me the power valve (included in most rebuild kits) and/or the accelerator pump could be the problem... does this help at all? hope so

well, on to future plans, since i brought it up, i definately want a longer duration on the cam, so that'll be replaced, i'll rebuild the heads, titanium valve springs, and a roller/rocker set-up, and i'll be running a super charger (still thinking of how much to put in, probably somewhere between 8-10 lbs) with a demon 750, or a holley 750 (still up for debate) so yes, i have some plans for this, and the 650 is getting replaced which is why i just want to rebuild it for right now, i still have at least a year before i can do that, so i'd like to have a reliable car until then, not to mention that gas already kills me (not bitching, because it's SO worth it) and i need to get something a little more eco-friendly (like a bike) before i do that... anyway, thanks again for the help... late

[Schrade]

My '87 TPI got over 29 mpg's after I swapped all the intake gaskets, and eliminated vacuum leaks. My '94 gets OVER 32mpg's, at 53mph.

You want all kinds of aftermarket mods? Like the other guy said, it is a NEVER-ending can of worm$$$. Some of the guys in the vette forums have $10K in carb'ed modded engines, with MATCHED components, and CANNOT get a proper tune. All because of simple problems that they thought would go away with a carb swap.

I promise - it HARDLY EVER works...

Serious - get the TPI set-up, TPI fuel pump, and put it in.

[MrPbody]

I was only trying to clarify. "Myth-bustin" is one of my goals. I grow weary of any car with a V8 being called "muscle car"... (:- Never intended you to take it personally...

I agree with cbec about the TPI system, but... You can't use a knock sensor in an engine with solid lifters. It will contantly try to retard timing... Some brands of roller rockers will create the same issue. The knock sensor can't distinguish between valve "clatter" and detonation.

I would recommend a smaller ProCharger if you really want a small blower. ProCharger offers the best of the turbo coupled with a belt-driven unit. We've learned to avoid BG carbs. They're nothing more than a genuine immitation Holley, so why not have the REAL DEAL? AED really does offer the best "out of the box" performance carbs around. And reasonable!

cbec,

I would question the tuning capabilities of those with the problems you mention. Carbs have been around since Christ was a corporal, and most of us have no trouble tuning for both power and drivability, provided one starts with quality components. As I said, I agree with you about TPI, but when "right", carbs make more usable power and at only a slight reduction in efficiency. For a stocker, no way carbs would be better.

Jim

[C4CorvetteGuy]

ok, basically this veered way away from my questions, not that i mind, i will keep all of this in mind when i get the money and time to upgrade, my only real question was, and it's my fault for not clarifying, what would the symptoms of a bad accelerator pump be (besides the implied lack of acceleration). in other words, from somebody who has experience with this particular size and make, would it make a whistling sound, or should it be fine to just rip the carb out (delicately of course) and rebuild it (with a "Holley" kit), or should i get some other parts? thanks, and i'll hit this place up again when i need some advice (probably march i get some extra cash) when i make the switch from naturally aspirated.

[MrPbody]

A bad accellerator pump will cause a "flat spot" or what could be called a "bog" (not really accurate, but for our purposes, "works") when you first hit the pedal. It won't necessarily cause poor accelleration. I would point more at a lean condition for that.

The "whistle" is usually from a vacuum leak. Air is being sucked through a small opening at a high velocity. This too, can cause the poor accelleration by CREATING the "lean condition".

After rereading your intitial post, I'm leaning more toward cam or ignition timing as the culprit. Have you tried adjusting the valves? This too, can create the symptoms you describe.

Put a timing light on it. Put a vacuum guage on it. "Read" the spark plugs. All of this will help narrow it down.

Sorry, didn't mean to take you off track.

Jim

[Schrade]

I'll say again that you should consider returning the motor to it's original configuration.

Get the factory service manual for the car. It has ALL the information you need, which will save you time and money, which you say you do not now have.

Then, if you do mods and build-ups to the base configuration, and something goes wrong, you know right away what is wrong. With non-original configuration, you will always be in the dark, as will anyplace you seek help.

You can probably sell your intake and carb for MORE than the cost of your intake, plenum, and runners, and come up with extra cash on top of that.

That's my

[C4CorvetteGuy]

about the timing... i thought of that already, but as mentioned previously, i have had no problems up until now... it was running great, and then it started to get sluggish, and then it was just messed up, the sluggishness and the worsening of it all happened within a week, which, from what you say, sounds like a vacuum leak... i've never had timing just go way out of wack in a week, i can run a timing light to it, but i haven't changed anything even slightly related to timing since i got it, so i'm thinking not timing. as far as going back to stock... while i don't have much money, i have way more money than time, i pretty much work aover 12 hours a day, and my weekends are almost non-existent, so to find time to sell everything, buy everything, and replace everything is not going to be worked in right now, i'm just trying to work with what i have right now, without being so cheap that it's not worth it in the long run... now about the vacuum leak, is there a way to tell about that without special tools, i don't have access to a full-service shop, so something i can do in my driveway would be ideal. further, would a rebuild kit fix that problem, or is that a completely seperate issue that i need to look at other parts. the whistling is coming from the carb for sure. could it be the seal to the accelerator pump? or possibly the power valve? i was planning on rebuilding the carb anyway, but i'd rather get everything i need at the same time, rather than trip after trip to the parts shop...

[Schrade]

Originally Posted by C4CorvetteGuy

now about the vacuum leak, is there a way to tell about that without special tools, i don't have access to a full-service shop, so something i can do in my driveway would be ideal. .

I think you're makin' the wrong decision. That's up to you tho'.

Here's vacuum leak detection procedure.

Loping idle is vacuum leak. 30 year old lawnmower, or a new car on the lot. If it sucks air where it ain't supposed to, it lopes.

Brick on pedal, block TB arm, anything to get a STEADY idle. 1200rpms or so... Feed starting fluid spray, A LITTLE AT A TIME, EVERYWHERE. Rev increase is the leak.

In a tough spot to pinpoint? Light a book of matches, blow it out while the heads are burnin', and feed the smoke to the leak.

It's gonna' be tougher with a carb setup (as will ALL problems), than with fuel injection, to find the leak (IF you have one). You have to make sure the spray doesn't get drawn in through the carb opening, at the same time, checking the carb-to-intake seating for leaks. Get the carb air filter on WELL for the test.

[C4CorvetteGuy]

sounds like a plan stan, thanks for the help, i'll tell you how everything goes

[MrPbody]

To avoid "false" readings, get a piece of 5" diameter "stove pipe" or dryer "vent" about 12" long. Put it on top of the carb with the air cleaner removed. While the engine is running, spray around the base of the carb, all the vacuum lines, including at the distributor (if so equipped) and around the heads where the intake is bolted. When the engine gains RPM, you have found the general area where the vacuum leak is.

cbec,

You REALLY don't like carburetors, do you? They aren't nearly as complex as you seem to believe. Not any more difficult to service than EFI, either. But I had the benefit of learning them FIRST, and not as an afterthought. Because virtually all modern cars use EFI, I can understand if you learned fuel systems since 1985 or so, you wouldn't be as comfortable with carbs as you are EFI. It took me a while to get used to EFI! No big deal, just not as tough as you seem to think. And in performance or race cars, carbs DEFINITELY have their "place". But again, I agree, EFI would be more appropriate in this case.

Jim

[Schrade]

Originally Posted by MrPbody

To avoid "false" readings, get a piece of 5" diameter "stove pipe" or dryer "vent" about 12" long. Put it on top of the carb with the air cleaner removed. While the engine is running, spray around the base of the carb, all the vacuum lines, including at the distributor (if so equipped) and around the heads where the intake is bolted. When the engine gains RPM, you have found the general area where the vacuum leak is.

cbec,

You REALLY don't like carburetors, do you? They aren't nearly as complex as you seem to believe. Not any more difficult to service than EFI, either. But I had the benefit of learning them FIRST, and not as an afterthought. Because virtually all modern cars use EFI, I can understand if you learned fuel systems since 1985 or so, you wouldn't be as comfortable with carbs as you are EFI. It took me a while to get used to EFI! No big deal, just not as tough as you seem to think. And in performance or race cars, carbs DEFINITELY have their "place". But again, I agree, EFI would be more appropriate in this case.

Jim

Hey there Jim...

Carbs are EASIER to tune and diagnose (if you know what you have) than FI. I know how to adjust mixture on any 2 or 4 barrel, like the best mechanic there is - PROMISE! Now sync'ing 2 x 4's or triple deuces I have not done, but I bet could get it right...

I am NOT a fan of swapping to carb because of failed diagnostics, which I have seen TENS of DOZENS of times in the vette forums, ESPECIALLY TPI's. And if you can't do it right, as apparently the other owner didn't, you shouldn't do it at all.

EFI does manage fuel better. My old '87 TPI got just under 30 mpg's at 55. My '94 LT1 gets OVER 32mpgs at 53mph flat highway. No carb could do that.


Edit: OP; see if you can find out if the other owner was in any vette forums, and find out his username. There could be a TON of interesting (and highly beneficial) info there on your car...

[MrPbody]

Once I learned to trust my instincts, EFI became quite easy. It's a lot like electronic ignition. It still DOES the same thing the old stuff does, just in a different manner. "Syncing" carbs is easy enough if you have the right tools (like anything else). I like the Pontiac "TriPower" better than Chevy's 3x2 and Dodge's "6-Pack". The Holley-equipped units NEED synced and each carb has a raft of adjustments. The Pontiac is much easier. The "end" carbs are just along for the ride until you hammer it. Their system is more like a "6-bbl" carb with 4 of them "secondary". You drive using only the center carb ("primary").

I always marveled at how efficiently the computer could control the mixture and spark. The fuel economy high performance cars are capable of today is nothing short of amazing. 400 horsepower, 27 MPG...

Jim