Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: Warthog1959 on March 11, 2012, 02:56:33 PM
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My engine has recently been rebuilt and is running well. However it is failing to engage into 4th and has to be put back into 3rd to engage drive. Is this likely to be clutch adjustment?
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No. The clutch will affect all gears equally. What was rebuilt and has this problem been there since the rebuild or happened after a while?
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No, sadly not, this sounds like a selector fork/drum or thrust washers between gears problem :(
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The entire engine was rebuilt, This is the 1st ride. This needed doing because there was a crack in the crankcase in front of the front sprocket. 4th gear worked before the build, however was subject to the occasional false neutral. All bearings and seals were replaced. Clutch pressure plate was replaced as this was functional but broken.
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Did you inspect the dogs on the gears while it was apart, also the wear on the selector forks (I would hope you paid particular attention to the wear patterns on the forks if it was previously giving trouble!) Were the gear clusters dismantled? was a thrust washer lost/omitted/broken up into tiny bits?
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No broken washers the gear cluster was inspected, but not Dismantled.. Possibly selector forks then? Can't say I would have been able to recognise wear on these.
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They tend to wear at the points where they actually contact the gears, also if someone has persistantly tried forcing gears in they can bend to the extent they wont move the gear across enough to engage :(
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Ok thanks. I'm not sure what to do now, selector forks don't appear to be available. Shame summer has just arrived.
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What model is the bike?
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It's an f2 super sport 78
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:o are you sure its fourth gear and not fifth gear that's not engaging,the reason i ask is, if you have left out the half noon rings that hold the gearbox bearings in place or you have used new bearings without the slots cut in them the slots are there to hold the bearings in place and stop any side movement of the gear cluster,then the main shaft will move over with bearings and the gear selector drum making the gears fail to engage >:( bad news is engine out again you might get away with just removing lower crankcase and work on engine upside-down
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We have an engine mount swivel to do that with. i didn't replace the bearings on the gear cluster. I did just get a set of selector forks and a shaft for an F2 for £22. didn't actually dismantle the gear cluster.
Is there any possibility that this is the selector drum. I have a spare used one of those also.
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Sorry yes it is 4th. I've carefully counted 1 down and 2 up, then nothing.
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Worth checking the seletor drum, also the lug on the selector fork that engages with the drum
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As the others say, it seems most likely to need access by splitting it but you should be able to do it in a day so still be up and running for the coming summer.
It does sound like an incorrect thrust washer/assembly mistake to have absolutely no engagement at all.
What happens if you try to get fith by clicking through without the engine running? can you get anything there and if so is there still no 4th when you come at it from that direction?
I know it's too late now, but I always click through the gears after assembly and before installing the motor. Just can't resist on bikes or cars as you can even feel any part engagement just using your hands.
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The selectors were checked before reassembly, went through the range. I can't get past 2nd when the engine is not running.
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That seems quite odd.
You usually have to rotate the output sprocket (turn the rear wheel) to get all the gears but it should definitely go through all of them.
Unless something brutal has happened to the selector forks they are quite strong, hence why we are pointing towards the gear spacing on the shafts as supect i.e. the correct gear is moved the right amount by the selector but the receiving gear dogs are too far away to be connected with.
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BOTH gearbox shafts need to be turning to get past 2nd easily, BEFORE i get jumped on yes it will frequently work just turning the back wheel BUT NOT ALWAYS
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Just remembered, the thust washers are integral with the gears on an F2 engine, so you wont have left them out, but they may have broken up :(
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The problem was the cracked crankcase repair. The weld was very slightly proud and prevented 4th from engaging. Unfortunate but no damage. Bike back together and running pretty good, apart from 1 of the downpipes running luke warm. The tests I did had me convinced it was the spark plug so changed all 4 with iridium's, but it is still not as hot as the other 3, The ignition is electronic so, I suppose it could be the coil or HT lead. Can one side of a coil fail? anyhow Thanks for your efforts. as a result I now have a used set of selector forks as spare. No bad thing.
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Try putting the lead/cap from number 4 onto number 1 and vice versa, if number 1 is still not right, it's not the lead! What condition are the H.T. caps in?
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They seem in fair condition. It is the 3rd down pipe from the left. I swapped the HT lead with 2 and the fault didn't move. I actually took the plug out and tested it for a spark in 4 and the plug failed. So I replaced all the plugs at once. BUT after a run out the down pipe is still tepid. It does not feel like it is running on 3 though.
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How does the offending cylinders new plug look compared to the others after a run?
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I'll check it when I next get chance to go out. Are you thinking fuelling? We did adjust the needles recently as it was running rich.
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Needles, fuel levels, partially blocked jet, just a case of going through it step by step and a process of elimination. I assume the valve clearances aren't too tight on the offending cylinder?