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85 RFVC 250 Boggs and dies after top end teardown and rebuild.

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Hi gang,

I had a head bolt let go on my 1985 RFVC 250 motor, so it started chuffing out through the head gasket. Up until that issue, the bike had been running fine, except for the slight off-idle bog common to most of these. I pulled the head and found that a previous owner had put the shorter head bolts in the barrel and used the longer barrel bolts in the head. They'd bottomed out when the torque was applied and this is where the stripping had started. Anyway, I heli-coiled the stripped hole in the barrel and reassembled with the bolts in the right places. Got 30 ft/lb torque on all barrel and head bolts so happy with that for now. I acquired a new pair of XR200 RFVC dual carbs and have used the linkages and seals etc from them to rebuild my 250 carbs, also adding a new set of push-pull throttle cables from the TT Forum Shop. I've left the jets as they were. I have also installed a r/h crank case cover from a model which has the oil cooler, and have mounted the cooler itself on the headstock.

After reassembly, the bike will start with choke as usual, but after a warm up, I ease the choke closed and it dies. I cannot keep it going with throttle alone, it seems to want to keep idling with a bit of choke and lets me do a little rev, but a hand-full just kills it as does closing the choke all the way. In frustration, I've tried the 200 carbs with their own jets etc and the symptom is the same.

Have I perhaps got the cam sproket 1 tooth out?

Suggestions etc welcome, thanks, Ray.

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Ok, I trawlled through the forum posts related to timing issues and found a post where someone said something like "these bikes will run with the cam one tooth either side of normal" but they didn't expand on the symptoms at all. I'm trying to get my head around what would be the effect of being 1 tooth out either way. The bike sort of snorts, almost a backfire, when I try to rev it and if I keep trying it will die but if I back off it will keep running at fast idle with choke.

If I have the cam 1 tooth too far clockwise then the valves will be fractionaly early, 1 tooth the other way and they will be fractionally late. Would either of these conditions give me the symptom I'm seeing?

My sprocket marks are as close to parallel to the head as they can get, with the flywheel T on the notch. They happen to be perfectly parallel when the flywheel F is on the notch...

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if the bike doesnt idle without choke, the pilot jet and circuit is plugged up.

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if the bike doesnt idle without choke, the pilot jet and circuit is plugged up.

X2

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I'd replace the Cam chain,,Sounds a bit out. I couldn't say for sure whether the fact that yours is lining up dead level when you're on the F would cause the problem. I'd replace it to rule it out as a cause,,Bit of backfiring and farting sort of indicates it could be a timing issue. Mind you a badly gapped spark plug will also cause that as will the carb possibility..Get a new Camchain,,check the plug gaps correct, check the Pilot circuit as well,..

Edited by Horri

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Cheers Horri, I'm trying to source a new chain now. Pilot circuit is clear but I'll gap a new plug and try that too. I spoke with a bike mechanic today who suggested low compression can also do this, stuck rings etc. My head is cracked so he could be on to something there but it's been cracked for a while. I've got a NOS XR200R head that I'm eventually going to valve up and use so that will remove the cracked head from the equation.

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Well, new plug correctly gapped made no difference. I got a hold of previous owner and checked that there were no timing idiosyncrasies on this particular bike and he confirmed not. Still waiting on new cam chain but meantime I've helicoiled two more head bolt holes 1 x 10mm and 1 x 8mm and I've transfered the valves to the new XR200R head I bought, and lapped them in. I've torqued it down on two head gaskets to make up for the fact that the combustion chamber is a bit smaller on the 200 head. With the cam, timing gear and rockers installed, it rotates without interference. I don't intend to run this bike hard and I think the oil cooler will also help. I cut the steel oil cooler lines off near the banjo fittings and replaced with 8mm copper pipe and easi-flo'd them back together so I could get a better path down the stem and in between the headers. The steel ones that came from the doner bike had all sorts of complex bends and did not work out the best. When the bew cam chain turns up, I'll reset valve clearances and see how she goes.

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Hi Ray, has she been fired up with the 200 head on her yet?

Does the right hand case with the oil cooler holes fit with no problems; I take it the original clutch cable was long enough to reach the clutch perch (it's further back to the left on the newer cases yes?)

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Hi Ray, has she been fired up with the 200 head on her yet?

Hi Chris, no not yet, I think the new cam chain will get here on Monday. With the old chain on, there was no slack, I guess the extra head gasket has raised the head that much. I may end up taking a link from the old chain and adding it to the new one to compensate.

Does the right hand case with the oil cooler holes fit with no problems; I take it the original clutch cable was long enough to reach the clutch perch (it's further back to the left on the newer cases yes?)

Yes, no problems with newer r/h crank case cover fitting, I believe it's from a '90 model. I angled the clutch cable from the headstock down the left of the head and around under the carbs. It used to go straight down the stem and along the top of the l/h crank case cover. The new route puts it over the rear of the engine right by the kick start decomp cable. I took the clamp and attached it to one of the engine mount-to-frame bolts. If I could figure out how to upload photos to this site, I'd show you!

Cheers, Ray.

Edited by XRay250

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I'm kicking myself that I even started this project. A couple of k's up the road I've just seen a '97 Baja XR250 (twin headlights), road legal, registered and warranted, 17000 km, for $2100 ono. I've already spent at least that on this thing... Ain't that the way it goes!???

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I got the bike reassembled and adjusted the tappets today before trying to start it. I found that the tappet adjustments have to be wound in a lot further with the 200 head because the combustion dome is much shallower than in the 250 head, so the valve stems are not as close to the rocker gear.

Even with two head gaskets, the compression was too high to kick it over with any degree of ease. It would do only one stoke per kick with me jumping on it and I'm 200lb. Normally I can kick it with my other leg planted on the ground. On the actual compression stroke, it fired and ran but the pinking was horrendous and I shut it down.

I will have to tear it down and have a re-think about how to use this 200 head on the 250. I might make some molds of the combustion chambers and try to work out the volume of the 250 chamber, subtract the volume of the 200 chamber and get the piston crown milled to give the remaining required volume. Using the two head gaskets did not provide enough compression space and also made the cam chain very tight.

Ray.

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Ok, tore the top down again and took some photos this time. The pinking was actually mild piston strikes on the valves, so even with the two head gaskets, there was not enough crown clearance when running. Here's the piston still in the barrel showing mild strike marks in the valve cut-aways.

bike004.jpg

Here's what the two heads look like, XR250 left and XR200 right, you can see how much smaller the chamber is on the 200...

bike003.jpg

Heres my piston after I sanded the crown right off with the bench sander and finished it with wet and dry... this should give me a couple more cc on chamber... the ding at mid 6 o'clock is what's left of a screwdriver mark I'd say, that a previous muppet was using to find TDC...

bike008.jpg

And here it is back in the bore after easing the valve cut outs a bit as well, with a dremel.

bike009.jpg

If you want to see the recoil inserts, check these out... 1st is 8mm lock-sert type and second is 10mm heli-coil type

bike007.jpg

bike005.jpg

I know it's not pretty, the Piston work I mean, but I just have to try!

Edited by XRay250

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