This Forum is now CLOSED use the link to get more details viewtopic.php?f=3&t=13924#p102587
By Creaky45
#94736
My bike is a 2005 Classic. Iron barrel bored to 535 with high compression piston from our host. Recently I bought an open silencer from them. I already had a POD air filter. The recommended jets are 125, P2, 30 and a 2.5 slide. Mr H was out of stock for the P2 so I am running the original but I can't read the numbers on it. (So I have 125, P0, 30 and 2.5) I presume it is P0. With the needle in the richest slot it runs great, any other slot very badly at low throttle. Yesterday I took it for a 95kmph plugchop but it suddenly slowed to a stop after 15 seconds at 95. When it stopped I found it had no (none at all) compression and suspected a holed piston. The plug was burnt white, very very lean. A plug chop at lower speeds had previously showed the plug to be perfect. Strangely after cooling down it got compression as normal, started first go and I rode home slowly, the plug looking good when I got home. I had a 130 main jet which is the highest I have so I put it in, test rode it at less than 75 and all ok. I will not go any faster yet. (bad weather and lockdown) It seems I have done no damage to the engine and the compression feels the same as before and it runs as before. Any hints out there regarding an even higher main jet or maybe I MUST get a P2. (I have neither at present and postage from UK is ridiculous)
User avatar
By stinkwheel
#94737
No compression though... That's not being caused by carb jetting and it's not a seizure, that's more like a valvegear issue. If you think about it, the only way it's going to lose compression short of a holed piston is if the piston rings are stuck in or a valve is stuck open. A white plug indicates super lean running. So I'd say a valve must have stuck open somehow because I wouldn't expect jammed piston rings to fix themselves.

On the original cylinder head on my 350 bullet, the valve seat came loose and kept dropping out of the head causing the valve to stick open periodically before clunking back in again -this woudl cause a total loss of compression. It was accompanied by a lot of popping from the exhaust when I closed the throttle because it was leaking slightly even when it was sat in place.

If it happens again, check the tappets. If a valve is stuck open, one of them will be very loose. Actually, maybe worth checking the tappets "hot" just to make sure one isn't too tight and holding a valve slightly open when the pushrod heats up.
By Creaky45
#94738
Thanks, Mr Stinkwheel, That's something I never thought of and it indeed sounds feasible. I will give it another plugchop when times permit and take my 13mm spanner with me to check the tappets if it stops again. The head is original but last time I had the valves out and reseated all looked ok. So do you reckon the needle jet, not being a P2 has nothing to do with it?I will keep you posted
By vince
#94739
Hi, when my exhaust valve stuck open it was like a seizure but without the zip noise but was preceded by an exhaust bang. The valve stem was too tight. Vince
User avatar
By stinkwheel
#94740
The needle jet shouldn't really affect wide-open throttle, it's in play between 1/4 and 3/4 throttle. Above that it's main jet.

It can help to put a piece of tape on the throttle mount (marked off in quarters) and a "pointer" on the twistgrip when doing plug chops so you can immediately see where the throttle position is. It's amazing how often you THINK you're on wide open throttle and actually aren't.
#94741
Check for adequate fuel flow through the tap, all the way into the float chamber. Some taps on Indian machines produce little more than a dribble and I have worked on engines which have suffered damage as a result, including one with a holed piston and others which have seized.
A 535 I worked on for someone had been continually tightening up, it had a Concentric carb fitted and going up from 106 to 107 on the needle jet fixed that, so it sounds fuel [or lack of] related in this case, too.
User avatar
By stinkwheel
#94742
Bullet Whisperer wrote:
Fri Jan 08, 2021 1:46 pm
Check for adequate fuel flow through the tap, all the way into the float chamber. Some taps on Indian machines produce little more than a dribble and I have worked on engines which have suffered damage as a result, including one with a holed piston and others which have seized.
A 535 I worked on for someone had been continually tightening up, it had a Concentric carb fitted and going up from 106 to 107 on the needle jet fixed that, so it sounds fuel [or lack of] related in this case, too.
From interest. How would a tightening/lean/overheating engine cause a reversible but total loss of compression? I can't get my head round the mechanism of failure.
User avatar
By Adrian
#94743
I think I'd want/have to strip the top-end and find out exactly what has happened mechanically.

As for a suspected holed piston, I had this once on a BSA B40, the Boyer ignition rotor cam adrift, and my attempt to re-set it accurately by the road side wasn't a good as I thought, as evidenced by the bike chuffing to halt a few miles later. I wasn't hearing anything that sounded like pinking, but sure enough, the piston was holed and there were little splatters of alloy on the end of the plug. You might want to double check the ignition timing while you're about it.

And while I am NOT advising anyone to desert our hosts, if you are where I think you are, then you might want to have a word with Mikuni Oz about the carb issues and see if they can help, given the Mikarb's ancestry and the delays/costs in getting stuff from here at the moment.

A.
#94744
Just recently, my 500 lost compression suddenly, (it was cold though), it turned out it was the cylinder head gasket leaking because the bolts were not tight, (neither were most of the others!)
Just something to check, is the head torqued down properly?
#94746
stinkwheel wrote:
Fri Jan 08, 2021 3:09 pm
Bullet Whisperer wrote:
Fri Jan 08, 2021 1:46 pm
Check for adequate fuel flow through the tap, all the way into the float chamber. Some taps on Indian machines produce little more than a dribble and I have worked on engines which have suffered damage as a result, including one with a holed piston and others which have seized.
A 535 I worked on for someone had been continually tightening up, it had a Concentric carb fitted and going up from 106 to 107 on the needle jet fixed that, so it sounds fuel [or lack of] related in this case, too.
From interest. How would a tightening/lean/overheating engine cause a reversible but total loss of compression? I can't get my head round the mechanism of failure.
I have witnessed it first hand - an engine loses power and tightens up under load, whipping the clutch in and backing off the throttle allows the engine to stop turning instantly. Waiting a few moments, the engine frees up enough to kick over, but with little compression - it may start, or may get going with a push start. Nurse it home. Back at base, find the compression has largely, but not completely returned, best strip the top end and have a look. Find traces of melted alloy from the piston skirt stuck to the bore at the rear of the cylinder and that some, or all rings are trapped by alloy which has wiped over them, but some compression returned as the rings wiped down the alloy stuck to the cylinder wall. Clean up the bore with coarse emery - there will probably be no scoring, assess the piston and possibly fit a replacement, or just fit new rings, after all, the material from the piston that tightened the engine is no longer present. Reassemble everything, checking for any possible causes of seizure, other than the piston having been too tight in the bore [if applicable] and off you go again, but at the ready to back off the throttle and get the clutch in more quickly than last time! Assuming a valid reason for tightening up was found and fixed, it should be ok in the same situation as when it tightened up last time. I use the Black Mountain climb to test machines after they have done this kind of thing and, generally, if they make it all the way up with no drama, they are good to go.

Shop for accessories at Hitchcocks Motorcycles