Fuel delivery issue

movingchicane

movingchicane

New member
We chased a stumbling issue all last year. Car would start out fine and as the race went on would start to cut out at mid to high rpm but only during the day when it was hot. Figured out we were getting air in the fuel line. Added a return line and haven't had any issues until this past weekend at rockingham.

So this weekend started out great. Car ran perfect during practice and qualifying. Started the race fine but about 15 minutes into my run the engine would fall flat on its face at the end of the front straight and at the end of the short straight before the first 180 degree right hander in the infield. In both places my car is running about 7500-8000 rpm. Pitted at 20 minutes and while my co-driver was strapping in I moved the fuel pickup from the left to the right side of the fuel cell. I was thinking maybe it was a pickup problem. Anyway it didnt make any difference, the problem continued to get worse until it would barely run at the end of the race. In retrospect that may have been a mistake because I dont think I got the fuel pickup shoved under the foam enough, and it may have floated up making a small problem a lot worse.

I will be installing a surge tank this week to rule out any fuel pickup issues.

Im pretty sure its vapor lock, just trying to get some ideas before the double enduro at roebling road this weekend.

Thoughts?
 
Is the fuel line properly shielded or routed away from heat?

The stock fuel line in my camaro was ran right next to the header... Used to drag race it and after an engine upgrade it started going lean between half and 3/4 track! new routing solved the problem.

Even seen FI systems with no return line vapor lock when it was hot enough out... had to push in the schrader valve to let the evapoated fuel out so the car would make the rest of the session!!!



The other issue would of course be fuel pump capacity...

P/U issues don't usually happen in a straight line, but weirder things have happened!
 
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What fuel pump? Helped a friend with an IT7 that had one facet pump and it would get hot and not pump enough fuel as the race went on. Do you have a pressure gauge near the motor to see what you are getting? We also had to wrap his fuel line to prevent boiling.
 
We use the stock fuel line which runs up the left side of the tub along the "frame rail" or whatever that piece of paper mache is called. The exhaust runs underneath the right side. The fuel pump is mounted in front of the left rear axle on the rear bulkhead. The only place where the fuel line is close to heat is when in runs up the firewall and into the carburetor.

Steve,

Its a Carter fuel pump, I think 72gph, 8psi max. Im running about 3 psi. I dont have a fuel pressure gauge in the cockpit but I stuck a guage on it before I put it on the trailer and it was dead on.

Thanks
 
jeff i thought you had that licked at daytona, with the return line?????
i think your fuel pump may be going bad,or may be an electrical issue, voltage drop when hot.
when you have the problem check for battery voltage at the fuel pump, also check for proper ground. also check fuel pressure and volume cold and hot.
good luck buddy!!!!!!
 
Three psi seems low to me but I've never check the fuel pressure on my rx7, its just a street car...

The IT7 guys will have to comment on this one.

Either way, I would mount a gauge on the cowl so fuel pressure can be observed while on track... Looks cool to :D
 
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steve s said:
jeff i thought you had that licked at daytona, with the return line?????
i think your fuel pump may be going bad,or may be an electrical issue, voltage drop when hot.
when you have the problem check for battery voltage at the fuel pump, also check for proper ground. also check fuel pressure and volume cold and hot.
good luck buddy!!!!!!
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I thought we had it licked too. I think i broke my radio screaming obscenities into the mic when it started missing, i know these symptoms all too well. I hate when stuff like this happens especially when the car has been running so well. We ran at daytona, roebling and kershaw this year and all of those races it was pretty hot and the car was flawless...engine wise anyway, everything else broke.

I actually replaced the fuel pump last year right before daytona so I would be suprised if it was bad but you never know. Battery is a month old and cranked strong after the race. Ill have to check the battery connections and grounds when I get to the shop. Alternator is charging fine. Ill check the pressure/volume this weekend.

Im gonna try and reroute the fuel lines and wrap them. Any suggestions on what to wrap them with? Also looking for any suggestions on lowering the temperature under the hood. Exhaust heat shield, etc...

Thanks for the help
 
CRallo said:
Three psi seems low to me but I've never check the fuel pressure on my rx7, its just a street car...

The IT7 guys will have to comment on this one.

Either way, I would mount a gauge on my cowl so fuel pressure can be observed while on track... Looks cool to :D
Click to expand...

According to my engine builder and lots of other rx7 gurus as well as what I have seen on the dyno is you need about 2.5 at the needle in the nikki carb. I did mount a fuel pressure guage on the dash last year when I was diagnosing the problem. It was temporary, I dont like fuel lines running in the car even if they are steel braided. Fire is the only thing that scares the crap out of me. On the cowl is not a bad idea

Thanks
 
Jeff- I'm guessing you still have vapor lock problem. Don't know where you bought your gas for the weekend, but if it was near Rockingham, there are a couple of possible problems. You need low volatility gas to avoid the problem, and the volatility specs for that part of NC are not as tight as in places like Atlanta or Charlotte (for air quality reasons). Second, if it was at a low volume station, you could have even had some left over winter spec (much more volatile) gas, since I think the changeover date is mid-to-late May.

I hate to give away one of my "secrets", but a final fix is to let your gas jugs sit outside for several hours with caps off. It's a form of do-it-yourself volatility adjustment.
 
Tom,

I had that thought too. The car was filled up somewhere in SC on the way up. The fuel we put in before the race was bought in Rockingham. Maybe I got some bad gas.

The fuel jugs were sitting in the garage which was pretty cool so it wasnt hot when we put it in the car. Ive taken that a step further and stuck them in a cooler before. :)

Thanks
 
Definitely run a fuel delivery pressure check.
I chased a similar problem, on a much different FI system; changed the pump and then found that the pressure regulator had an eroded valve seat and was incapable of maintaining proper pressure when fuel volume demand was high.
Don't know where the fuel pump is located. If its at the rear, next to the cell, then vapour lock on the suction side is unlikely. If its up front then consider moving it back if rules allow. But keep the regulator as close to the carb as possible so the low pressure line is short.
Bill Miskoe
 
irondragon said:
Definitely run a fuel delivery pressure check.
I chased a similar problem, on a much different FI system; changed the pump and then found that the pressure regulator had an eroded valve seat and was incapable of maintaining proper pressure when fuel volume demand was high.
Don't know where the fuel pump is located. If its at the rear, next to the cell, then vapour lock on the suction side is unlikely. If its up front then consider moving it back if rules allow. But keep the regulator as close to the carb as possible so the low pressure line is short.
Bill Miskoe
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Pump is in the rear. Regulator is right before the carb. Definately gonna do a bunch of fuel pressure tests this weekend.

dickita15 said:
Are you getting fuel in all 4 barrels of the carb when you rev it?
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Yep, everything is spraying like it should.

Any suggestions on what to wrap the fuel line with?

Thanks
 
For a cheap and dirty way hardware stores sell rolls of fiberglass pipe insulation that you cover with foil tape.
I assume the car runs fine now that it has cooled off.
Keep in mind that pressure gauges often fail unless they are very good ones.
By the way 3 psi is right.
Also the dyno showed that with a stock fuel line we last a lot of power above 7000 with 2.5 – 3 but overwhelmed the floats above 3 psi. The solution was to increase the size of the tubing from the pump to the regulator.
 
In a Niki carb volume is more important than pressure. 2.5 - 3 psi is good. More is bad. I run a 1/2 inch line from the pump to carb and use the stock line for the return line. Never had a fuel starvation problem.
 
Is there any way for you to reduce your heat load? Do you have a stainless header? If so, I found that a fiberglass wrap named "Inferno" survives, and the header survives as well (Even running a race on secondary ignition only!)

If it only occurs at the end of the run, you are probably on track. It couldn't hurt to check your float level, though. I can't believe how insanely sensitive these carbs are to float level !!!

Reducing the underhood temp is key; I was on the dyno yesterday, and found that carb was getting too hot to touch. No matter how cool the fuel is, we were getting breakup at high RPM when the carburetor was too hot.

For the curious, it was an electric dyno, saw 121 HP at 7600 - 8000 RPM at the wheels, had one run that hit 125 HP at the wheels at about 8400 RPM, but what Dick said occurred, for the most part; above 8000 RPM, indicated AFR went lean and power dropped off. HOWEVER, observers heard ignition break up, so I'm not sure root cause is fuel. The lean indication could be due to misfire, due to insufficient spark at high RPM. Coil? Who knows, I couldn't afford to keep diagnosing ...
 
dyoungre said:
Is there any way for you to reduce your heat load? Do you have a stainless header? If so, I found that a fiberglass wrap named "Inferno" survives, and the header survives as well (Even running a race on secondary ignition only!)

If it only occurs at the end of the run, you are probably on track. It couldn't hurt to check your float level, though. I can't believe how insanely sensitive these carbs are to float level !!!

Reducing the underhood temp is key; I was on the dyno yesterday, and found that carb was getting too hot to touch. No matter how cool the fuel is, we were getting breakup at high RPM when the carburetor was too hot.

For the curious, it was an electric dyno, saw 121 HP at 7600 - 8000 RPM at the wheels, had one run that hit 125 HP at the wheels at about 8400 RPM, but what Dick said occurred, for the most part; above 8000 RPM, indicated AFR went lean and power dropped off. HOWEVER, observers heard ignition break up, so I'm not sure root cause is fuel. The lean indication could be due to misfire, due to insufficient spark at high RPM. Coil? Who knows, I couldn't afford to keep diagnosing ...
Click to expand...

What make dyno?
 
Ignition breakup would make the afr go massive rich due to unburht fuel entering the pipes, We had an issue years ago with not enough air entering the carb,solved with a hientz air filter system. Heat shields and ducting air to the exhaust system to blow the heat out seems to help. Connected a hose to the back of the radiator to right in frt of the #1 pipe and ducted out under the car. Cool the carb and air entering it as well as you can by sheilding it.
 
Thought I had said - whoops - .

Mustang Electric dyno.

As for breakup going rich: True, if using a 3 gas analyzer, misfire would show excess rich HC / CO. Using a UEGO, though, you are looking at residual oxygen only. Breakup / miss would cause a false lean. Now that I think of it, I should have run on primary ignition only, that way a miss would be a full miss, easier to diagnose. Come to think of it, best run was with the air cleaner base, but no filter / top (using that old 'flying saucer', the racing beat housing with K&N filter). Ran that way to see if my brake ducting 'cold air intake' was causing a restriction. Answer is YES !!!
 
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Thats what happened to us. Chased a skip/break up/run problem all weekend. Finally got pissed slammed the hood and said we need laps for a finish. As the car drove off it ran way better as I stood there with the air cleaner cover in my hand..:shrug:
Guess thats racing. Dan
 
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