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Won't start after fitting big bore. (Jetting) ?

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Just finished fitting a 436 Athena big bore kit and am struggling to get it started. Was running great with the stock bore, i had already fitted an FCR a few months ago jetted to suit a FMF Q2, 155 main / 200 main air jet / 45 pilot / no pilot air jet / EMN clip 3 / 2 turns out on the fuel screw. and thats how it is now. should this jetting be near enough for it to start. It tries to start but will not keep running. there's a spark at the plug fuel's getting to the carb. ( UK bike no vacuum tap ), and the valve timing is correct. Any ideas.

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Nothing to do with jetting.

If you are sure about cam timing, try a new plug and fresh fuel.

If it has sat around for a while, check the carb for blocked jets.

Neil. :bonk::naughty::D

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Found what was wrong a valve shim had moved under the bucket some how and was holding the valve open. starts ok now not sure if it's done any damage though.

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When it did not start, did seem like the starter was laboring like a high compression issue, ie, the valve hitting the head "Phhtt unghmmm phhht unghmmm" or did it simply whirl away 'phhttt, phhtt'.....

Sorrry for the technical terms. I bet you are ok. How does it run, feel the power?

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When it did not start, did seem like the starter was laboring like a high compression issue, ie, the valve hitting the head "Phhtt unghmmm phhht unghmmm" or did it simply whirl away 'phhttt, phhtt'.....

Sorrry for the technical terms. I bet you are ok. How does it run, feel the power?

Just whirling away no laboring, befor i started it turned it over half a dozen times by hand and it felt OK. Don't know whether to strip it down again and have a look, but not sure if i can get a head gasket for a 94mm bore.

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Don't know whether to strip it down again and have a look, but not sure if i can get a head gasket for a 94mm bore.

Only from TT store, I could not find one in the UK, without special order.

A leak down test will tell you if you have done any damage, or if not available to you, just do a normal compression test, it won't be very accurate because of the auto-de-compressor, but post the results and I'm sure somebody will know what a healthy engine should give.

Neil. :bonk::naughty::D

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Check the valve.. Choose your method, leak down or remove the head.. But check the valve. It takes so little of a strike to vend the valve,, then at 7, 8, 9, 10,000 RPM when IT DOES POP THE HEAD OFF, not if, but when.. your looking at a crank, rod, piston, cylinder, and head replacement. If it's not bent.... it cost you a gasket or leak down test and a few days.. If it is bent you'll save a $1000 in replacement parts.

There are short cuts and some things you can get away with not fixing or checking.. Valves on this motor are not one of them. I'm a long away from my home computer, or I would post pics of the

Aftermath of a OEM two piece valve becoming two pieces again.

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I'll bet you are ok.

It runs and sounds great real smooth. I' m going to put it on light duties till i can sort out a head gasket. Fingers crossed.

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I'll bet you are ok.

Always nice to bet with some else’s money :D

Hey like all things internet, use what you read, modify it, or lose it. Your choice.

Having been inside one or 2 DRZ motors, having rebuild three that had destroyed motors due to valves issues. Knowing that two of those had a perfectly good "looking" set of valves in the head, right before the valve broke... I know what know,,,and what I would do,,,, And suggested the same.

But to each his own :bonk: Do what makes sense to you :naughty:

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The possibility of a bent valve would have stopped me from even trying to start it, after I had found the miss-placed shim.

The DRZ does not have the best reputation for valves anyway, and the pictures of Bronco's and 10guys (among others) engines post broken valve, was not a pretty sight.

And to have it happen in the UK with no easily or quickly available spares would be my idea of hell.

Neil. :bonk::naughty::D

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I am leaning towards the side of I don't see how the valve could be bent.

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I am leaning towards the side of I don't see how the valve could be bent.

See there’s the difference, you don’t know how it could be bent, and I know several ways it could be. So you would not check, and I would. Nothing wrong with your decision based on what you know. I simply would choose different. :bonk:

At TDC with the stock piston, do you now what the valve to piston clearance is with a properly adjusted valve? I do :bonk:

Google the phrase

“Interference design motor “ 😛

In all likely hood, based on what he said,,,, it’s probably fine. :D

But at the cost of a gasket, or even taking it to a shop and having a leak down test done (assuming he does not have a tester of his own) To me, in my experience, it is worth the time and cost.

But I admit, bikes are my toys, not daily transportation.. SO it’s easier for me to make that decision. As well as I can have most any part I need overnighted if it’s really worth the trouble to me. Some of our overseas friends may not have that as an option.

:naughty:

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I am leaning towards the side of I don't see how the valve could be bent.

With the shim out of place, the valve will be opened further, and may well come into contact with the piston.

I'm not sure what the piston valve clearance is, but it is a lot less than the thickness of a shim, and a slightly bent valve wiil, under spring pressure very quicly become a two piece valve again.

And if you think that you would hear the valve hitting the piston, think again.

A valve that is leaking a little, would probably not stop the engine starting, but would very quickly lead to a burnt valve seat.

Neil. :bonk::naughty::D

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You all are telling me stuff I already know. And yes I have dealt with bent valves before in more then just motorcycle engines.

Edit, I forgot to put in a smilie so yall don't think my panties are in a wad.:bonk::D:naughty:

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Is this why RHC valves are recommended during a big bore upgrade? Are the stock valves crap?

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Are the stock valves crap?

Some would say YES, and with good reason.

I always wanted to change mine, but £££ stopped me, however my valves did 20,000 miles, most of those with a Hot Cam inlet and I had no problems:excuseme:

Neil. :bonk::naughty::D

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