I need help!

I recently had to have one of my rocker arms shafts(Scat) replaced because it snapped. One of the push rods was also bent(Chromoly) and I replaced it with an aluminum one. The mechanic also balanced the carbs and adjusted the valves.

When I got the car back from the mechanic, It ran fine for about ten miles, then suddenly it seem to run on only 3 cylinders. It would idle smoothly, but when rpms reached 3000+, the motor would sputter and miss. Also, It wont rev above 5500 rpm. I took it back, the mechanic changed the plug wires, re-adjusted the valves and balanced the carbs.

This time the car ran fine for about 15-20 miles before it fell back into the aforementioned problems. What needs to be done? This sucks because it spanks ass for a little while, then it sucks after a little bit.

Any possible solutions?

I am no VW engine expert but I would suspect the aluminum pushrod. Since Aluminum has a different CTE (coefficient of thermal expansion) than steel (the other chromoly pushrods), I believe that the aluminum pushrod is lengthening more than the steel and is changing the valve clearances when it gets hot. If the valves are adjusted cold then the engine warms up to operating temperatures the Aluminum pushrod gets longer than the steel. I would either get all aluminum pushrods or all chromoly.

It could also be partly due to the different spring constant of the aluminum vs steel pushrods. When you compress (or tension) a straight column, the column will compress (or extend) slightly. The amount of deflection (delta) is PL/AE where P is the load, L is the length, A is the cross section area, and E is Young's Modulus for the material. In your case P is the maximum load in the pushrod. The spring constant is P/delta which is equal to AE/L. Since L is the same for both the steel and aluminum the only difference is the AE term. E for steel is 29,000 ksi, and for aluminum E is 10,100 ksi which is about 1/3 that for steel, so for the same area A the aluminum will have about 1/3 the spring constant. As an example it will take 3 times the load to get the steel pushrod to deflect the same as the aluminum.

However, I think you problem is more temperature dependant since it occurs after the engine warms up.
Message Edited 6/16/2003 11:04:05 AM)

(Message Edited 6/16/2003 11:06:34 AM)

You broke the rocker shaft!?!?! Whoa!

Sounds like you have some sort of coil bind. You using the correct ratio rocker for the cam? Are the proper shims installed under the rockers for the use of lash caps, or non-stock valve adjusters?

Aaron

If you snapped a rocker shaft, you may also want to inspect the rest of the valve train for damage such as bent valves. I wonder what rpm you were turning whan the mishap occured?

Thanks for replying guys. Sorry it took so long to reply back. I don't think I was turning more than 5000 rpm when the mishap occurred. I could possibly have a sticking valve? That could cause the valve train to bind up and snap the shaft and bend the push rod.

The theory on the aluminum pushrod sounds interesting, but I've never heard of anyone having expansion problems with them.

I think I'm gonna get it looked at by a different mechanic; but first I'm going to do the simple things first (change the fuel filter, make sure the jets are clean, verify electric pump is doing its job, clean the k&n element) just so I don't waste money and embarress myself.

As soon as I can get a day off from work, Im going to try the easy stuff and post my results.

Anthony -
I think what the issue may be is that from your post it sounds as though you are mixing chromoly & aluminum and that will relly cause big problems from what I have heard

Joe

Anthony;
The gist of my rather lengthy first reply ( I have been called a storehouse of worthless information) was what Joe also pointed out that you seem to be mixing Aluminum and Chromoly pushrods, which I theorized was not a good idea.

I took my car to Durkee Performance in Hawthorne, CA(approx 2 miles from LAX) and discovered that the last mechanic that I took the car to made a mistake. Although the last mechanic was competent on stock motors, he did not have alot of experence with high performance motors.

According to Mike Durkee, the last mechanic set the valve lash too tight and then adjusted the carbs to that incorrect valve setting which resulted in my problem. The last mechanic charged me $150 when I took it back to him, kept my car 2 days, and the problem returned within 10 miles of driving. When I took the car to Mike Durkee, I had the car back in 4 hours and payed only $50.

I've used Mike in the past for an exhaust leak I had and I've always known him to be fair, honest, and very competent. I was impatient (you have to make an appointment 2 weeks in advance because he is so busy) and took the car somewhere else.

Mike has been building high performance motors for well over 20 years. He has a 9 second turbo Ghia in his shop, that he still races, that he built in 1986! and the motor has never been broken or torn down and rebuilt. He is currently in the process of building 16 different motors ranging in size from 1600cc to over 2300cc; both turbo and non. He told me that he has a 1915cc turbo that would work well in the 550 chassis and put the car solidly in the 10's in the quarter mile.

Mike doesn't know I'm plugging his business for him, It's just that I'm happy with his service and knowledge and would like to pass it on to the spyder community.

If anyone is interested in talking to Mike, let me know here and I'll post his info.