Cartrax

1984 Crossfire Corvette - Getting TBI backfire with moderate throttle under load. Only a soft gradual throttle will eliminate the backfiring.

Reading it may be valve timing or a bad valve or timing or ECM or..... With the following compression readings can I eliminate valve timing and a bad valve?

Readings cold, throttle closed, all other plugs in.

First Cycle/Fourth Cycle.
#1 110/152
#2 110/152
#3 100/150
#4 97/150
#5 98/151
#6 90/140
#7 100/150
#8 100/150

Other Specs
355 Rebuilt with ~ 300 miles on it
75# Injectors
Renegade Manifold
2" Bored TBI's
SBC Street Fighter Cam (Mild Mod- Have Specs)
9.72 to 1 Compr Ratio
Regulated Fuel Pressure
High Perf Fuel Pump - Not sure of specs

Have EBL Set-up - but have not run VE Learns yet - thinking the the backfire issue needs resolution first.

alvanwie

Increase Accel Enrichment. Should cure the problem.

Cartrax

Thanks for the reply, assume that means you don't believe the valves are an issue with these readings?

thomas1976

I could not tell from the compression readings. But did you already check the spark plugs, fuel pressure and double check initial timing?

Cartrax

I did. Ran through these and others (TPS, Map, O2, Vacuum) more than once. Got frustrating. Why I was thinking valve timing. I'll have to try that accel enrichment adjustment. My laptop battery died so it will be a bit.

Was hoping the compression readings eliminated the potential of a valve issue. Thanks.

Schurkey

A better test of valve sealing would be a leakdown test using compressed air injected into the spark-plug hole. ANY leakage past intake or exhaust valves (hearing air escaping from the throttle body or tailpipe) is not acceptable. ANY air escaping into the radiator (bubbles in the cooling system with rad cap removed) is unacceptable. SOME, minimal leakage into the crankcase (hearing air escaping out of the oil-fill hole in the valve cover) can be acceptable, but too much is not acceptable. A real leakdown tester can put numbers to the leakage helping you determine what's acceptable leakage into the crankcase.

thomas1976

If no signs of oil combustion on the spark plugs, then it is probably timing and/or fueling related.

Cartrax

Thanks, if I can't resolve it I'll have this done.

Cartrax

None noticed. Hoping to resolve with AE, timing, or VE learns. Also need to balance the TB's better but I don't believe that would cause the backfiring. Thanks.

Cartrax

Progress - still waiting on a laptop battery (China express) but I built a manometer and balanced the TB's. In the process I jacked the idle up to about a grand - not intentional. Thought you couldn't do that ... anyway, will have to drop that later when I change the injectors and now I have a balancing under my belt.

During the test drive I noticed less backfiring. Guessing that's cuz I'm pumping more fuel into the manifold with the higher idle - that's probably a good indication it is a fuel enrichment issue. No?

Ronny

iTrader: (1)

Verify timing.

But with the manifold you use and the 2.0" TB it needs a lot more AE. I would start with 10% more AE for TPS and MAP both. You may see improvement. From there add another 5% and again see how it responds. You may end up with a lot more needed yet.
you have a very large intake and TB's. It all needs to be wetted. When you left off gas or depress the throttle Vac decreases and it goes dry.

Be aware the EBL allows you to add more at certain RPM's. You may find it needs more <2000 rpms but not > 2000 rpms.

I had a similar set up on my 84 CF.

thomas1976

Going by the description of the new engine, I would first set up fuel pressure and a new bin with the basic adjustments, according to the new hp output, if it has not been done yet.

These links contain input that may help you if needed, to set up the new bin
https://www.thirdgen.org/forums/post...4-post178.html
https://www.thirdgen.org/forums/post...0-post207.html

Cartrax

Thanks for the advice/direction, guys - exactly what I'm looking for. Picked the right forum. Will post results...

Cartrax

Took it out today and it seems to be running worse. Missing and backfiring on the slightest load. This without touching anything.

Reading the referenced posts - not understanding much yet - so many options - where to start. Will have to hit the books & Dynamic EFI's tuning intros again. Will keep your suggestions in mind as I'm going through things but I have a ways to go before I can even appreciate "em. Will be back...but not for a while. Probably need to start with some basic VE learns - if there such a thing. Thanks.

thomas1976

Once your laptop is operational, post a complete list of the engine specs including cam, so we can help.

Once you a have a base bin, set up for your engine, it will be the good time to do VE learns.

Cartrax

Will do.

Cartrax

Laptop working had to replace CMOS battery also. Here's the specs again with cam details.

355 Rebuilt with ~ 300 miles on it
75# Injectors
Renegade Manifold
2" Bored TBI's
SBC Street Fighter Cam
SBC Street FighterHydraulic cam 288°/292° .450/.460 lift. Duration @ .050 lift 224/224° degint/exh, lobe seperation 114°, advance 2°
9.72 to 1 Compr Ratio
Regulated Fuel Pressure
High Perf Fuel Pump - Not sure of specs

Have EBL Set-up - but have not run VE Learns yet - thinking the the backfire issue needs resolution first.

Will hook up to EBL later today. Recommendations on where to start? Thanks.

Ronny

iTrader: (1)

Lean....May need a whole lot of AE fuel due to plenum size

thomas1976

Need some additional information:
Cam intake and exhaust centerline, cylinder head flow numbers and valve size, exhaust, intake manifold flow numbers

RBob

iTrader: (1)

Make sure that the fuel pressure is stable and holds under load. Using a pressure sensor and logging it is the best way.

As for the AE, start by multiplying the MAP & TPS AE PW tables by 1.2 (20% higher).

Then multiply the MAP & TPS AE Filter tables by 0.9 (10% lower). This will increase the duration of the AE event.

But make sure that the fuel pressure is stable, otherwise the tune will go in circles.

RBob.

Cartrax

Definetly intend to adjust the AE and test results.

Cartrax

Not sure I have this. I had the engine done. Will look...

Cartrax

Thanks, pressure sensor? Where can I get one? Is this just a gauge?

thomas1976

I use the Actron 7817, with the T on the feed line next to the alternator, nothing fancy, but it works.

Cartrax

Just replaced injectors today with a set of 90's I bought last year. Started fine - no road test yet.

Actually not real sure now what the old ones where - was told they were 75's.
Part number on one of the old one's indicated it was a 40, the other had no part number. Anyway, planning on re-setting fuel pressure to 18psi - per BPC calc should yield about 400 ponies. Plenty for me - any comments?

Checking timing in the morning then will set up new bins then start working suggestions from there - think this makes sense if not let me know - thanks.

Cartrax

OK, looks like an item I can afford. So you take readings while driving and log the results?

thomas1976

I put the gauge under the wiper arm, to verify if pressure drops under load. Pressure must stay constant.

Ghettobird52

Does that gauge that comes with the kit reach to the windshield/wiper arm? Is it safe? Im using that actron unit as well its very nice and installed it in same location as you. That thing would NOT fit behind the Throttle body.

thomas1976

Mine is just long enough to go from the T next to the alternator to fit under the wiper arm, though I bought it around 8 years ago, not sure they are still made the same length.

thomas1976

Sounds like progress. Flow matched injectors is a must.

Cartrax

Timing verified @ 10BTDC - Scalar Int Time set to 10BTDC
Changed BPC vs VAC and BST-BPC vs Boost to BPC Calc Values
Added 20% and Reduced 10% Values Noted Per Above
Set AE->TPS% Double Map AE to 100% per Tuning Instructions
Saved values and new BIN
Flashed new Bin
Test Drove....

Still getting backfire on accel with slightest load. Little performance change noted.

How can I verify values were accepted by ECM?

Also, no way of testing stability yet - part on order though.

RFmaster

In WUD you can always download BIN file from ECM into a new file and perform BIN file compare in the TP.

Just a sanity check - how do you check your base timing???

//RF

RBob

iTrader: (1)

Use the PromID in the BIN to keep track of them. It is only used for this purpose and is shown on the main WUD displays.

RBob.

Cartrax

Engine at Op temp, bypass disconnected, timing light. Engine starts right up - if that's an indication of anything. Anything else I should be checking? Thanks.

Cartrax

Thanks will look at this. I'm thinking maybe I should recreate the BIN starting from scratch just to ensure nothing crept in there to create issues. Recommend a base/starting BIN?

Cartrax

Additional specs(?) I found this looking through some papers. It was handwritten on the engine rebuild invoice. I did not deal directly with the rebuild shop and parted company with this mechanic not on the best of terms. (He wanted to carb it against my instructions.)

Exactly as written - not sure if any of this helps.

Aluminum Heads

CAM 050 224/224 114° Lobe Sep 2°
In .450 Lift
Ex .460

Heads 245 at .450 Lift
64CC Flat Piston

9.72 to 1 Compression

Another shop pulled the engine and installed a new balancer - they could have distorted the cam timing. Does the engine need to be pulled to check this?

Car has long tube headers installed.

Ronny

iTrader: (1)

When I flash in a new bin I typically name it by that/todays date. Sometimes I add after date ae or pe or propgain whatever the change was. Like 060514propgain.bin. If the flash goes well you will see the green acknowledgement message and the dash light will blink. Be assurred it is running on the new bin.

RBob

iTrader: (1)

Check that the spark timing advances once the EST/BYPASS is reconnected and the ECM is reset (key-off for ten seconds). The timing mark on the damper should move toward the passenger side of the car.

RBob.

Dominic Sorresso

Did you replace the stock harmonic balancer?

Cartrax

Yes it was replaced.

Cartrax

Thanks, using unique naming conventions. Flashes are taking (green ack.). Learned to run a compare, it indicated values per ECM BIN have changed. I noticed you don't get exactly the percentage of change you input - assume that's normal - ?

Dominic Sorresso

Yes, there are constants in the conversion formulas that limit the amount of change. VE tables,as an example, must change > .39%. Having said that, I have found that I can 0 out a value, save the bin, then replace the 0 w a new value even though it was < .39% than the original VE. Don't know what the value of this may be.

Cartrax

It does advance (moves to passenger side) with EST reconnected. Judging about 6 to 10 degrees.

Revving it made it jump initially another ~6 then it stopped moving when revved after that. Shouldn't I see it advance more as I kick up the RPMs?

Ronny

iTrader: (1)

Maybe not much adv since not much load

Dominic Sorresso

Have you verified that dizzy is positioned correctly? That you are not one or two teeth off on cam drive? Just askin.

Cartrax

No, never did. Will need some guidance on this. Where do I go?

Cartrax

OK, thought that might be the case.

Cartrax

Hit today. I now understand the "is it safe" question. Plenty of hose to fit under wiper if I create a junction just east of the alternator.

Can you elaborate on your set up and how the test is performed or reference a thread? Where does that run-off typically go as you're driving?

Thanks.

Dominic Sorresso

Take the D/S valve cover off, turn crank until balancer mark is at 0 or maybe 6d TDC. #1 I/E valves should both be closed. Now look at distributor rotor. Is it pointing at # 1 cylinder on dizzy cap? It should be right there. If not, you're distributor is not set properly.

petrolhead

Going back to basics, I've seen plug wires do exactly what you are describing several times. Try a spray bottle with the engine running in the dark and watch for sparks. Dist. Cap can do that too.