Symptom: backfire on the overrun

There have been some comments regarding this happenning and solutions from carb adjustments to various other ailments. Thought I'd share what I found. My 750 combat has been doing this for more than a year. It's not really that annoying, so didn't dig into it. Recently it started blowing oil out the left exhaust port (leaking around the exhaust nut). Inside the cylinder didn't show any oil burning so it seemed the only cause could be a loose valve guide. It was. I'm getting the head back tomorrow and both exhaust guides were loose needing oversize replacements. That makes sense now as to the backfiring. Air was leaking into the exhaust ports through the guides.

Why after only 14,000 from total rebuild did the guides get loose? I think due to the pushrods being too long. I'd like not to have the problem again in 5 years, so would like some feedback.

Illf8ed,

Let me preface by saying I’m no valve guide expert.

Nevertheless, I suspect that longer push rods will change the geometry of the valve train and cause the associated forces to be resolved differently. I would expect to see a bit more scrubbing at the rocker and valve stem interface as well as bit more side loading on the valve stem and guide. However, I would not expect these forces to be severe enough to loosen the valve guides. The forces resulting from longer pushrods are similar to those induced by higher lift camshafts. And I’ve not heard of any valve guides loosening owing to higher lift cams.

Instead, I believe that your valve guides were marginally loose since the time the head was last overhauled. Perhaps the guide bores became damaged when the old guides were pressed out and new ones installed. On the other hand, perhaps the outside diameter of the new guides was a tiny bit undersized.

I’m uncertain of the benefits, if any, but I always use Loctite whenever I install new valve guides.

Jason

backfire on overrun

Jason,

I like that answer. After buying some "short set" pushrods from Smith Brothers recommended by Dr Hiller then finding they were the same length as what I have makes me think the ones already in the engine may have been shortened already. Does anyone have a set of stock pushrods they can measure? My exhaust (short one) is 7 19/64" measured with a machinists scale.

David:

Sorry about your experience with Smith Bros. I'll get back to you on that "heavier" Cro-Mo pushrod thing, but basically they're not at a place in the valvetrain where weight is critical. More later.

Now, as for your guides, I agree with Jason. Sideloading severe enough to loosen the guide in the head would likely show up first as serious valve stem wear.

If your head is being done by someone who knows what they're doing (proper clearances, etc) and they're using good materials (Kibblewhite Ampco 45 guides, etc) - you should be trouble-free. That said, I believe there's a guy named Murphy who's had extensive experience with all things mechanical... So much, in fact, that he's come up with a law...

Good luck.

pushrod standard length

David,

No sweat on the Smith Bros pushrods. It was worth the $18 to see if they were shorter. I just got the head back on yesterday and the valve adjusters have very little poking out the valve stem side. May have to see if someone can get the steel end off the pushrods and remove .045" - .050". I went to the gas station just now and it was misfiring on the way back. The plug wires are nearly worn through rubbing against the head steady, thought that might be a problem. At least taking the gas tank off the head bolts got checked again for torque - front two under the exhaust were already loose.

Big ride next Sunday with the club, so I want no problems.