Losing Power...

[jpetillo]

My fuel cell has no internal leaks in the pickup line and fittings.

I'll check for flatness of the carb top, have a new gasket (rebuild kit), will replace the valve, will replace my nifty glass fuel filter, and the fuel hoses. I have a new pump and will replace that, too.

Oh, and won't I over tighten the top of the carb.

Thanks for the opinions! John

[jrv]

Thanks for the replies.

I'm looking for long shots!

John,

Here is a long shot. What are your temperatures, especially head temps if you have them? Everyone in this thread has run you through the fuel and spark trails. If this is happening late in the race and comes and goes I can think of one weird, and cycling thing. You may have a hot exhaust valve: it would blow gasses by, cool off and deliver power and then go through blowing again. As for corners and Gs this idea does not work.

Our original VWs had valve clearances with the exhaust 0.006" and the intake at 0.004" Our Vees are all at 0.004". You may try setting your exhaust gap a bit larger on a hot day and see if it makes a difference. If you do a valve job look at the intake and exhaust valves and seats and see if there is any difference.

As I said, this is a long shot.

Thanks,

Jim Vaseff

[jpetillo]

Jim, Thanks! Yes, that does sound like a long shot. I'll add it to my notes, though. Even if the valve stayed open a bit, I don't think it would lose as much power as it seemed to. I don't know the temps, exactly, but I checked the gauge when she came in and it was probably 220-230 at most. I have 240 marked and it wasn't near that. John

[jpetillo]

I replaced the fuel lines, the pump and filter. I then removed the carb top and as several of you suggested, it was not flat at all. Brian H, yes, I can now see why you shouldn't tighten it too much. It was clearly warped down at the four corners. The glass plate and 400 grit emery cloth did the job in a jiffy. I don't know if this was it, but I also changed the valve and gasket, of course.

We'll see if the problem is cured this weekend.

Thanks for all the help.

[Rickydel]

Well, you've changed so many things, I certainly hope your problem has gone away, but you may never know what it specifically was.

I had the exact same problem. Car ran fine for most of the session, but towards the end, it would cut out, burble coming out of right hand turns.
As Brian said earlier, my issue was a pin hole leak (very hard to find. I had stainless braided line.) in the fuel supply line that goes through the fire wall.

Here is why it happens only on right hand turns. My fuel line goes through the fire wall on the right side of the car. Around left hand turns, especially when the cell is full, the lateral G forces pressurize the inlet side of the supply line even though the pump is pulling. Around right hand turns, the G forces are pulling fuel away from the supply side of the fuel line on the right side of the car. The pump is pulling a vacuum as well. So when you have negative pressure on the inlet and outlet side of the supply line which may have a pin hole leak, air gets sucked in.

Good luck.

Ricky del

[jpetillo]

I agree that I changed so many things that I'll never know. But, since it's an issue that may only happen at the end of a race - I would possibly not know in test/practice/qualifying sessions until the race.

About the pinhole situation you described, I guess it would have to be a large enough pinhole to allow the float bowl to empty. I changed almost all of my plumbing because of possible air leaks and fraying (old) lines.

Thanks, John

[jpetillo]

I had no losing of power in turns this past weekend. LRP is next, and we'll see if it's cured there as well.

Thanks for all the help!

[NotMoss]

"Re: Losing Power..."

"Post by halifax on Tue Sep 08, 2009 8:31 pm
Since it was mentioned as a diagnostic rule out earlier in this thread, what are the causes of a jumping tach? Mine gets very bouncy at the higher RPMs."


Re: Jumping Tachs

Mine does the same thing above about 4000 or so. I've heard that this is a common shortcoming in VDO tachs (which mine is) of a certain age. A fellow vintage racer and I compared notes at Road America this past weekend and the AutoMeter on his Zink is totally stable when mine is flopping all over the place.

Thankfully, I have an SR tach lite (unpaid plug) that keeps me honest.

[brian]

Unstable gauages are the result of two possibilities. The movement in the gauge is worn or the signal is intermittent. The dampening circuitry in some tachs may be failing as well. The only way to check the tach itself is to install in another car and see if it still jumps around. Or, the next time to go to the dyno take the tach along and wire it in. Checking the signal is another challenge. Assuming all the connections are good, as I said in a previous posting, the dist can be an issue. Points bounce or worn shaft bushings can make the signal very erractic. Swaping the dist can test this component. Finally, I've seen coils put out bad signals as well. I've had this problem for years and finally have gone to a magnet and halo switch to drive my computer. Our ignitions are very "dirty" and don't put out clean signals.

[SR Racing]

At this day and age.. (and rules) use the Pertronix points replacement. You get another 1000 volts of spark (not that we need it), a better timing signal , no maintenance, no bounce and a better signal to the tach. We have been using them for 5 years in the FST and most recently in the FV's. It's a no brainer. (I would not recommend anything but the Pertronix since we have seen failures with the Compufire and other brands.)

[NotMoss]

Unfortunately, Pertronix won't help with my vintage car (illegal).
Any other suggestions before I buy an AutoMeter this winter?

[SR Racing]

Nope. As Brian said, most anything in the primary circuit can cause the tach to go erratic (points, condensor, distributor). Or just a bad tach.

Jim

[Matt King]

I wouldn't rule out the condenser. I have had similar problems on an old BMW motorcycle that I traced to a condenser and a friend of mine had a nearly identical problem as described here in his FF at the 40th reunion race that disappeared immediately after he replaced the condenser.

[sharplikestump]

Way to go, Ricky! I was reading thru the replys to see if someone nailed it....I think you did. I believe if there was a camera trained on a see-thru filter, it would varify exactly what you stated. I have watched it on the dyno. Pulling the inlet hose, plugging one end, submerging the hose and pressurizing it (lightly), bubble city!
Mike P.

[jpetillo]

Mike,

Not to say that this didn't happen, but getting bubbles of air in the supply line would just mean a bubble of air into the bowl. That would not be a problem unless the bubble was big enough and the bowl was emptying that fast that it emptied out. Do they empty that fast?

I would think that a pinhole could not have allowed that much air in, but I would believe that we were sucking air from the pickup on the right hand side being out of the fuel during the corner. We had about 1.5 gallons left.

John

[sharplikestump]

J,
That is exactly what happened, and I suspect it to be MANY pinholes. I pull my fuel from outside the dyno cell, through the wall, into a glass-bodied filter, and on to the fuelpump. When the demand reached a point, it would create sufficient vacuum to pull enough air to shut the engine down. This has actually happened several times, and this is with the cloth reinforced VW brand fuel line. Can't say I saw this with 1000 VW STREET engines, so I have to suspect that Avgas (which is all I use on the track as well as on the dyno) may aggravate the situation.
I'll stick with my theory....I think that was the problem.

[jpetillo]

I appreciate the comment. I did change all my fuel lines as a result of air holes being mentioned as a possible cause, so I thank you.
Of course we will never know since I swapped and dealt with everything that was suspected by folks on the forum (all together the cost was small fraction of the cost of a ruined weekend). John

[sharplikestump]

J,
Couldn't agree more. There are times to be analytical and sort through the puzzle, and there are times to just shotgun the dang problem and get back to racing. Ya done good.
Mike