I have just done a top end rebuild and replaced 3rd countershaft and main gears (see newbie post). Prior to the rebuild the bike started 1st time, everytime. Now it wont even fire. I have strong spark at the plugs, no fueling problem and have checked and rechecked my cam timing marks. Valve clearance set as per manual. When I crank the starter the engine seems to blow back through the carb and then backfires but no proper firing. I am no newbie to engines but this is the first Harley I have rebuilt and I have to admit to being stumped. It seems as if the ignition is out by 180 degrees but I have tried absolutely everything and had not even a sniff of firing. The bike is an '81 ironhead sportster but has points (that strange crossover year) Any suggestions?

 

Pull the plug wires and plugs and check the compression. By what you describe, I would check the pushrod adjustment and/or the cam timing. If I had to guess it sounds like your intake valves are open when the plug fires.

 

Yep, I thought of that and checked the cams this morning. All timing marks lined up. I did think I may have put 1 in the wrong way round but I cant see how that could be done. I have checked and double checked everything but have obviously missed something. Knowing my luck it will be something very simple that I have looked at a hundred times but its been 2 weeks and I really am banging my head against a wall. The last thing I want to do is pull the motor and start again.

 

Compression, timed ignition and fuel is all you need to run. One of them is bad. I'd still start with a compression check.

 

I have to admit the compression felt very low when I was turning over the motor to find the timing mark. Problem is I have new pistons, rings, reground valves, new gaskets etc etc. If its losing compression then it could only be valve timing and that means my cams must be out. I dont even know what to think anymore

 

What do you have for pushrods and lifters? Are they solids and adjustable? You could have them hung open if not adjusted correctly. Pull a plug and put your finger over the hole, turni it over by hand and see if you can hold back teh pressure. This isn't a conclusive test but if you can, you have a compression problem. IMO.

 

done that and you are right. Now all I have to do is find out the reason for it. Many thanks

 

Maybe off the subject a tad, but did you change the starter or the battery to non harley one(s).

This was last year at BikeWeek with S&S on hand. My S&S 124 motor wouldnt fire with either a spyke or one other high torque starters. When I put in an Harley regular starter and Harley battery took out an an interstate battery it turned over just fine and boom it ran. The problem was that these high torque starters werent spinning the motor fast enough. I didnt believe it till I saw it with my own eyes and sure enough first round she kicked off. If its unrelated sorry, just thought it might worth sharing.

-harry