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Discussion Starter #1
Bike: 2000 Honda Foreman 450 S 4x4

Problem: backfiring after deceleration at all gears, rpms and speeds and running really hot. The backfire is a loud shotgun sound.

It also is really hard to start. Always have to choke it and push started button a few seconds. It used to start right up if i sneezed on the starter button.

It is running really lean by looking at plug. Bright white and clean electrode. Soot on the bottom below the electrode and ceramic.

Absolutely NO smoke.

What i have done:
Riched the carberuator by dropping needle clip, bigger jets and more turns out on the pilot jet screw still running hot and backfiring after deceleration. None of this changed the lean condition.

I've checked the timing sprocket and chain at TDC compression stroke (right after the intake valve closes). All of the marks line up: the case with the T in the window, line on recoil cage lined up with case triangle and the valves had play in them.

compression check- 90 PSI dry and 120 PSI with oil placed in spark plug hole

Leak down test attempted but when I hook up hose to spark plug attachment it spins the crankshaft slightly. If i hold the recoil starter nut it will spin the other way. Even with 20 PSI on the gauge it will do this.


I am needing help.
 

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I knew someone that had this exact issue on the same machine . Header pipe would glow orange within minutes of running . Did all the same things you mentioned . He said it ended up being a missing o-ring on/in the carb causing a lean condition that he couldn’t fix with any amount of jetting
 

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Discussion Starter #3
I've tried multiple carbs and it does the same thing regardless of the carb that is in it.
 

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Adjust your valves
I had the same problem and found my exhaust valve was to tight causing it not to fully close
it was was hard to start and would backfire on decel
My problem cleared up after adjusting valves
 

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maybe the valve stem is bent or sticking ------- maybe the connecting rod is bent , did that engine ever ingest water ---- maybe boot between carb has a leak ---maybe a crack on the diaphragm of the carb ---
 

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Discussion Starter #6
Adjust your valves
I had the same problem and found my exhaust valve was to tight causing it not to fully close
it was was hard to start and would backfire on decel
My problem cleared up after adjusting valves
I've adjusted the valves several times and that didn't help. I will try again and report back.
 

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Discussion Starter #7
maybe the valve stem is bent or sticking ------- maybe the connecting rod is bent , did that engine ever ingest water ---- maybe boot between carb has a leak ---maybe a crack on the diaphragm of the carb ---
The crank and connecting rod are fairly new. I rebuilt the motor two years ago and it has given me fits ever since.

Carb boot to head and air box is new.

I've used several carbs and it still does the same thing
 

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The only other thing I can think of that gave me fits like this is, I was one tooth off on cam gear.
I had a tensioner that seemed good but found it would "bounce" when running and allowed chain to jump a tooth-got lucky to catch it before it got any worse
Had to run it with front cover off to catch how it happened, as I triple checked timing before assembly and know it had been correct, replaced tensioner and corrected timing and ran perfect for many years
Possibly pull front cover to check alignment of cam gear dot in relation to case marking at TDC
Man I know how it feels trust me, but you will figure it out I am sure
 

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Discussion Starter #9
The only other thing I can think of that gave me fits like this is, I was one tooth off on cam gear.
I had a tensioner that seemed good but found it would "bounce" when running and allowed chain to jump a tooth-got lucky to catch it before it got any worse
Had to run it with front cover off to catch how it happened, as I triple checked timing before assembly and know it had been correct, replaced tensioner and corrected timing and ran perfect for many years
Possibly pull front cover to check alignment of cam gear dot in relation to case marking at TDC
Man I know how it feels trust me, but you will figure it out I am sure

i'll look into that. I replaced the tensioner and chain when I did the rebuild but it guess it is possible the tensioner could be bad or maybe when i've looked at it i couldn't see that it was a tooth off.

Any idea why the piston is pushed down when i do the leak down test?
 

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Compression seems great and I believe your cam has a decompressor that opens the exhaust valve a bit until it reaches 450 rpm, so normal readings when cranking with battery are are bit lower
If your piston moves a bit I wouldnt worry as long the valves are still both closed ( rockers have play) as you are just trying to see how much and where its leaking
If you want to lock the crank by putting a bar thru slots in recoil cup so it cant turn either way but it usually involves c-clamps, getting the position just right, curse words and too much fiddling for me.
If rockers have play- thats good for me
I usually just listen with a hose to my ear for a leak- oil fill hole for crankcase, exhaust for exhaust valve and carb bore with throttle plate open for intake valve to see where the hiss/leak is
Honestly if your compression is good and this machine does not smoke I would not think you have too much of a leak
past the piston anyways
I usually only did a leakdowns on 2 stroke engines to find leaks for bad crank shafts seals and the like, then would use soapy water spray and the like to see leak
If it was me I would return to stock with a good clean carb, as so many variables get out of wack fast and muddle things up trying to pinpoint a problem
I know you said you tried many carbs etc but you might have a leak at carb slide diaphram I always put a light coat of white lithium grease around the lip to help seal, but slide leak issues usually also result in bog/loss of power as well
Dont get disheartened we'll figure this out
 

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Discussion Starter #11
All of my carbs are older and I want to be 100% certain that they are not the cause. I'm going to buy a new carb so that I can rule that out.

Question: the carb for a 2004 Honda Foreman 450 FM is $228 and the carb for a 2000 Honda Foreman 450 S is $330

Shouldn't they perform exactly the same? Any problem with me getting the 2004 for my 2000?
 

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Discussion Starter #12
Comparing the carbs for a 1998-2001 Foreman 450 to a 2002-2004 Foreman 450

Side by side comparison of the microfiches:

What the 2002-2004 has that the 1998-2001 doesn't:

an extra vent port thus several extra tubes, joints, clips

The only major difference other than the extra vent port is the newer version has a different part number on the float, float valve and the throttle linkage cover.

All of the jets have the exact same numbers.

What to you guys think, save the extra $80-100 and get the 2002-2004 Carb for my 2000 Foreman 450?
 

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I would not buy a brand new carburetor but that’s just my recommendation
I would rather get a quality rebuild kit from Shindy or Moose
or similar Not cheap China junk
Do not buy one of the $30 cheap China carburetors on eBay you’ll end up with more problems as they’re pretty hit or miss for reliability
If you tear down your original carburetor and get a couple cans of carburetor cleaner and some compressed air and couple toothbrushes there’s not much you can’t fix on them
Unless the float bowls are heavily mineralized from water or corrosion
That’s just my two cents from my experience over the years


Sent from my iPhone using Tapatalk
 

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Discussion Starter #15
I used three carbs. Two were OEM cleaned in ultrasonic cleaner and shindy rebuild kits. The third was a brand new Chinese carb.
All three presented the same problems.

each time the symptoms got better for a little bit. On one of the rebuilds I almost ran 100 miles before the first backfire but then it started doing it twice or more in a 10 minute ride.
Now ever time I take it out even around the neighborhood at mid gear and speeds. Every time I slow down from 30 mph or down shift from 4 or 5 gear it backfires like a 10ga shotgun.
 

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The newer 450 carb will work but you'll have to redo the fuel line. The newer carb the fuel inlet is more of a vertical position and the older are more horizontal (if I didn't get them mixed up). Have you sprayed carb cleaner around the carb/intake while it was running to see if it changes the engine speed at all? There should be no change unless you get close to a vent line on the carb that might be bad or missing.

I'll try to check in to keep up with this thread. I've moved on to another forum so I just do a random browse on here from time to time. Messages do notify me though so it still gets my attention that way, as you found out.
 

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Since your issues are the same regardless of carb used, I doubt you'll find the solution there. My feelings are that the top end is somehow to blame. I don't believe there's a decompresser on these engines, otherwise the tested compression would be lower than what you've seen. If there isn't an intake air leak or exhaust at fault, then you'll have to tear down the top end to find out what's up.
 

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I beleive at this point that all those carbs could rule out it being the issue
Were the good carbs tested on another known good machine by chance?
Most likely it would seem to be an air leak with the lean plug and backfire
Is your muffler stock and could one of those backfires been strong enough to blow out your baffles?
I have seen bad primer pump ball bearing valves in the float bowl and primer pump diaphram leak or fail
and under heavy throttle actually pull a fuel spray through the brass spray bar
I have a video of a running machine showing a primer stream form spraybar when you blip throttle-crazy but
it would be rich I would think, but might cause a backfire with excess fuel

Just for clarification and not to confuse other members these camshafts in fact do have a built in decompressor
I just put a camshaft in 3 days ago (picture from 98-01 Honda Foreman 450 Factory Servive Manual)
 

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I used three carbs. Two were OEM cleaned in ultrasonic cleaner and shindy rebuild kits. The third was a brand new Chinese carb.
All three presented the same problems.

each time the symptoms got better for a little bit. On one of the rebuilds I almost ran 100 miles before the first backfire but then it started doing it twice or more in a 10 minute ride.
Now ever time I take it out even around the neighborhood at mid gear and speeds. Every time I slow down from 30 mph or down shift from 4 or 5 gear it backfires like a 10ga shotgun.
If it gets better after the rebuilds and you got 100 miles out of it once then I’d still point to the carb or an intake leak . Your missing something simple . These machines are more prone to backfire on decel but shouldn’t do it all the time . Exhaust leak would also cause a backfire and as mentioned if the exhaust is coming apart it will need more fuel to run right . I know you said you tried going up in jetsizes but by how many ? Did you do both main and pilot jets?
 

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Discussion Starter #20
Last night I went to a buddies to get the carb off his 450. The carb is 1 year old OEM Honda carb and his bike runs perfectly.
I’m going to try it out and see how mine performs with it.

About a year ago my stock muffler’s baffles broke and were rattling around so I put the Armortec stock replacement muffler on. Here is the link to the exact muffler

bike.https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.com%2Fulk%2Fitm%2F273269756651

I thought my broken baffles were the issue causing backfiring so I replaced the muffler but that didn’t fix it.

Once I get my buddies carb on my bike I’ll run it to see what it does. If it still behaves the same (backfiring and hot) I’ll spray carb cleaner around the boots (even though both the boot to the head and air box are OEM Honda and less than a year old. Then I’ll have someone hold a shop vac in blow mode and spray soapy water wall over the muffler, header pipe and head.
I’ll report back my findings.

Thank y’all all so much for your advice and help. I really appreciate it
 
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