So Close!...
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@SpookDog I guess you could make a TZR pipe fit but is there a reason you want to?
Also the TZR (2RK) engine is very different to the DTR; much lower compression (I can remember blagging a go on a mate's TZR and thinking it'd blown up when I kicked it over) and the CDI unit is different as well as a few other bits.
Not wishing to shoot down your idea, a lot of lads in Portugal build DTRs with underslung pipes, I'm just curious
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Not for any long term reason. My ‘big one’ is falling apart. I’m looking for reasons that would cause poor scavenging of exhaust gasses (Ive been getting bad knocking/misfires for a while when coasting @ low throttle) I really just want to substitute another pipe for testing. I have a 94 TZR pipe laying around...
On a simular subject I just checked my reed valve and it is not fully closing (on 1 side petal) when ‘relaxed’ out of the bike. I don’t know much about them other than they are a one way valve, not sure if they should be sprung closed, or forced closed by the piston pressure when it goes down, or both!...
If it’s not holding the mixture in the crank case it could maybe really fuck it up and cause misfires, which in turn murder head gaskets...I wish I was a bit smarter! 🤪
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@HOTSHOT-III I run underslung, wheelie machine
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Hmmmm?! I just checked the Haynes manual and found out the ‘stopper height’ for the reed valve on the DTR125 is supposed to be 6.8mm, TZR 8.3. Mine is 11mm! I might just of found at least part of the cause of my ‘pipe banging’ which in turn could be the cause of my gasket problem...
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Just found a good working replacement reed valve with good neoprene rubber seals and flat petals, also with correct stopper heights! Just remeasured the one that was fitted and it is 11mm one side and 13mm the other! A far cry from 6.8!!...
I really am hoping that this was causing the misfires, exhaust banging and in turn the misfires was causing the head gaskets to fail...
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Coming to this conversation a bit later, but I thought I would add my two penny's worth.
Get the head and barrel skimmed professionally, using glass isn't the best option. I know a lot of people will say 'I do it all the time and never get an issue' (I have done it myself, when I can't get to use my milling machine). I know, If you do if correctly then it can do, but it will never be as good as in a proper machine shop. It's not just about having a flat surface, its about having a slight rough finish for the head gasket to grip to.
Check the squish band measurement, if it less than 1mm then this could be the issue, although I doubt this is the issue as lots of different heads and barrels have been used.
If you don't have the carb heating pipes connected to the carb, then they should be at least connected from one banjo to the other. so they is always a flow of water from the head to above the thermostat, ie bypassing the thermostat, to bleed air.
Check the timing is correct?
As stated before replace the rad cap. I would change the thermostat as well.
If the reed valves are not sealing/ closing, then replace them. If you are using standard reed valves then use the stock 6.8mm gap. If you have changed them for Boysen reeds or other after market, then you can open up the stops to give better performance, but they must still seal when closed. If you hold then up them up to the light and can see day light then either the reed block is damaged, or the reeds are knackered.
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I’m actually having a really hard time finding an engineer. Most have disappeared during COVID and half those left are cowboys. The only one I’ve found was not good. The local bike shop managed to fuck up a helicoil. I think I’ve managed to find one localish. Then again I’ve got a good barrel that needs a rebore, so I might just send it and a 3mb head of to PJME who I know do good work that can be trusted. It’s just having the cash and dealing with a 2-3 week turnaround...
I’ve got the reeds covered now. I’d love to work out the surface area of a 28mm circle, then do the area of the reed valve when fully open. The reeds don’t need to be any bigger than the carp at fully open. I’m mathematically challenged tho’
The timing isn’t supposed to be adjustable, but that doesn’t mean a previous owner hasn’t drilled/slotted out the stator mounting holes. It’s something I’ve been meaning to check but not gotten round to...
Any any all advice/ideas are welcome, cause I’ve been twisting my melon over this one!
Cheers bud!... -
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@SpookDog I still think there is an underlying issue here that's causing the barrel to blow the gasket. I've never seen this issue so prevalent in a DT before and I've worked with some proper dog of an engines that ran sweet as a nut when it was done.
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@SpookDog seriously think your massively over thinking this!
I've used many Athena head gaskets with no issues, shouldn't need to use any type of sealer on the gasket either. It's either something like your piston is hitting the head, or your cooling system isn't pressurising properly. I'd be looking very close at your radiator. Have you checked your water pump actually works? -
@SpookDog Surface area of a circle is pi (3.142 approx.) x (radius squared). Radius = diameter/2 so for a 28mm carb (14 x 14 = 196), x 3.142 = 615.75 square mm. If you have a barrel that's had loads of rebores you can also accurately calculate its cc (or swept volume) using this method, then multiplying by the stroke.
Surface area of the reeds when fully open is a little more tricky as the fully open reed petal produces an opening consisting of a rectangular part (at the very end of the petal) and a triangular part (along both sides).
I guess you could take care of the rectangular part by multiplying opening width x 6.8mm x number of petals.
There's also a triangular part to calculate which would (roughly) be 6.8mm x length of the part of the reed petal which actually opens. Then you'd have to divide it by 2 because it's the area of a triangle, or just leave it as it is and treat that figure as the total surface area of the triangular part per reed petal (because there's one of these down each side). Still not exact because the reed stops are curved so I'd iamgine the reed petals would follow their contour when fully open but it would give you an idea.
I think though that your fitting reed stops which are close to the specified height will improve the way the bike runs; opening up reed stops is one of those things people experiment with in their youth to try and make 125s faster (don't ask how I know lol). Rather like the previous owner of mine who fitted a massive main jet, then set the float height too low so it was trying to suck on a fire hose with a half empty float bowl
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@SpookDog I really don't think an o ring will solve your problems. Your problem almost certainly lies within the cooling system. Sticking/seized thermostat, defective water pump or defective pressure cap. You can easily eliminate the water pump by removing the hose and check if it's pumping and the thermostat by removing it and put in a pan of water on the hob and check the water temperature when you see it open.
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@SpookDog memo
Head gasket fitted. Radiator maintaining coolant at correct level. Temp good @ 140miles...
Poor Symptoms: still stumbling, misfires, knocking @ throttle just cracked open/idle cruising speed...
Find cause,
To check: exhaust manifold & spring seal. Coil. Flywheel side seal. -
Anyone used hi temp rtv silicon to seal a spring mounted exhaust?...
Rem: 210 miles, temp still good, especially for ambient temp. Coolant level still at filler neck bottom...
Still missing/stumbling (popping & knocking) quite badly @ low revs...
Checked airscrew, also carb & inlet for air leaks: all good...
Checking stator side seal before swapping out coil & CDI & source coil, pickup, & sidestand kill switch solenoids (2 of) & other ignition electrickery ...
P.S. Will try swapping out exhaust if necessary...
Edit:
Bodge sealed the rotor side seal with silicon and a smear of grease, also silicon sealed the side cover to make it as airtight as possible: running a lot better at low coasting/cruising speed without knocking/missing. Longer trip needed to be sure, definitely suspect the seal is leaking air. Would of expected oil leakage tho’... -
Tested the bike with bodged seal: what a difference! No popping, knocking & missing at all! Good smooth resolution of revs all through the lower end like I’ve never had!..
I never thought about the seal before cause it was replaced new with the bearings when I first built it. Teach me to buy fleabay seals
Will do the hole and self tapper screw trick and replace with a good used original I have a few of now...
P.S. Copper RTV silicon seems to work very well on the spring mount manifold!...
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Removed and replaced the seal. Took 20 mins. Didn’t need to make any holes, just popped it out (carefully not to scrape the crank) with a small flat head screwdriver. Other drifted in easy with a smear of grease...
Will be test riding bike later...
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Still popping & knocking a bit, but on a much narrower margin, and then only when hot...
Not the result I was hoping for. It is a lot better, but not 100% The change was so profound I was almost certain it was the culprit...
The copper rtv silicon is holding up really well on the exhaust manifold though
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I’m @ 535 miles with no head problems. Radiator is full, cooling is perfect...
Copper rtv still doing fine on manifold...
Swapped coil out with no difference. Bike still popping & knocking bad when it gets hot. I’m still thinking that it’s an airleak somewhere on the block. I don’t think it’s the pilot jet because it’s fine when cold, which would be when the mixture needs to be rich enough (& is!) Waiting on a new genuine seal (flywheel side) from PJME. Got a new genuine clutch side already to go. I’m not gonna take chances on pattern parts again if possible...
I can’t find any signs of 2T leakage on the crank casings join. I did wonder because I used red hematite instead of silicum sealant on the join...
Airscrew is very responsive for the first time ever. Will check the pilot jet again, but am convinced it’s not the problem. Is the same with different carb...
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