carb or timing? which way to turn! - Honda ATV Forum
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post #1 of 11 (permalink) Old 04-16-2019, 12:33 PM Thread Starter
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carb or timing? which way to turn!

Alright. Here we go! Bike is a 98 Honda foreman 450s (a beautiful faded yellow finish I may add!). The issue is as follows. Bike runs rough at idle. Very rich. Sounds like a lawn mower and blows black. I went in the carb and gave it a real good clean using compressed air and cleaner. Reinstalled it and had the same problem. My second thought was a sticking choke so I triple checked that and it seems to be seated and sealing as it should. I took off the small inspection plug on the right side and found two marks on the flywheel that line up with a mark on the case. At this point both valves are closed and I have proper valve clearance. I turned it over by hand and both valves seem to be opening and closing smoothly and completely. I took it for a ride and it seems to go pretty good cruising down the road and under load but anything slow it spits and sputters and takes some finesse to keep it running.i can see the vacuum secondary jumping a good bit at idle.if I hold it with my finger to keep it from jumping it smooths out and runs ok. All the Jets are the correct size. Needle jet is correct and set to spec. So do I focus on the carb or is it sounding like timing jumped a tooth. I got the bike like this. I hate to start ripping the front off only to find everything is fine. Thanks for any ideas you have.
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post #2 of 11 (permalink) Old 04-16-2019, 01:30 PM
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Have you tried to adjust the fuel mixture screw? Easy thing to check, directly underneath the carb. Just make sure the engine is warm when adjusting.
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1998 Honda Foreman 450ES
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post #3 of 11 (permalink) Old 04-16-2019, 02:14 PM Thread Starter
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Thanks for the reply. I have adjusted the idle mix screw. I started at two turns out. Then made my way to completely closed by 1/4 turns. I only noticed a small difference when it was almost completely closed. By small difference I mean the rpms slowed a bit and was a little more likely to shut off but not by much.
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post #4 of 11 (permalink) Old 04-16-2019, 02:29 PM
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It could be a vacuum leak between the carb and the cylinder head. Other than that, it could well be that the valve timing has skipped a tooth and you'll just have to dive in to investigate that.

2015 TRX500 Foreman
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post #5 of 11 (permalink) Old 04-16-2019, 04:53 PM
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Alright. Here we go! Bike is a 98 Honda foreman 450s (a beautiful faded yellow finish I may add!). The issue is as follows. Bike runs rough at idle. Very rich. Sounds like a lawn mower and blows black. I went in the carb and gave it a real good clean using compressed air and cleaner. Reinstalled it and had the same problem. My second thought was a sticking choke so I triple checked that and it seems to be seated and sealing as it should. I took off the small inspection plug on the right side and found two marks on the flywheel that line up with a mark on the case. At this point both valves are closed and I have proper valve clearance. I turned it over by hand and both valves seem to be opening and closing smoothly and completely. I took it for a ride and it seems to go pretty good cruising down the road and under load but anything slow it spits and sputters and takes some finesse to keep it running.i can see the vacuum secondary jumping a good bit at idle.if I hold it with my finger to keep it from jumping it smooths out and runs ok. All the Jets are the correct size. Needle jet is correct and set to spec. So do I focus on the carb or is it sounding like timing jumped a tooth. I got the bike like this. I hate to start ripping the front off only to find everything is fine. Thanks for any ideas you have.
you need to look for the letter T , NOT TWO MARKS ( LINES ).

'89 trx350D foreman 4x4...'85 vf700c magna, '03 trx450fm foreman 4x4 , 2015 Yamaha waverunner fx cruiser ho, '08 vt750c2 shadow spirit
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post #6 of 11 (permalink) Old 04-16-2019, 05:57 PM
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Think it goes 2 marks, then F, then T. If using the pull starter to turn the engine the marks go by pretty fast....

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post #7 of 11 (permalink) Old 04-19-2019, 07:30 AM Thread Starter
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Thanks for the ideas. That's what confused me about the lines is I read to look for the f and t but I don't see those marks through the sight hole. Just the lines. I spent a good half hour looking just going round and round before I got a second set of eyes and still just the lines. Kinda confusing. I've got some time today so it seems like I'm just going to dig in and hope for the best.
post #8 of 11 (permalink) Old 04-19-2019, 07:33 AM
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I think starter valve or air cut module on carb. OEM carb for that model?

Starter valve oring must be conical and not worn or you will not cut the enrichment circuit
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post #9 of 11 (permalink) Old 04-19-2019, 01:25 PM
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Thanks for the ideas. That's what confused me about the lines is I read to look for the f and t but I don't see those marks through the sight hole. Just the lines. I spent a good half hour looking just going round and round before I got a second set of eyes and still just the lines. Kinda confusing. I've got some time today so it seems like I'm just going to dig in and hope for the best.
the letters F and T are there !!!.

'89 trx350D foreman 4x4...'85 vf700c magna, '03 trx450fm foreman 4x4 , 2015 Yamaha waverunner fx cruiser ho, '08 vt750c2 shadow spirit
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post #10 of 11 (permalink) Old 04-21-2019, 03:41 PM Thread Starter
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I wondered about the carb more also. The o-ring on the enrichment looks ok but certainly not new. It is the oem carb. I'm not familiar with the air cut valve. Is that the vacuum controled diaphragm on the right side of the carb?
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