Re: Mototrans 250 Scrambler restoration: she's a runner!

[themoudie]

Aye Al,

As Colin advises, inspect EVERYTHING!

Don't rob "Peter to pay Paul", if the engine ran using the original carb, do a straight swap of carbs, to see if the carb could be at fault. Swapping carb components just gives more variables! If it runs with a rough old carb, then you know the fault lies with the new one. Also, just because the carb is new, doesn't mean that there is no swarf or obstructions in the drillings. I have seen the air bleed drilling on the inlet side of the slide smeared and resulting in no tickover, small throttle response. Check every orifice and use a magnifier/watchmakers loupe to inspect.

May I also suggest a cooler plug, NGK BR7HIX, rather than your NGK BR6HIX. I have found that I was getting blistering of the central insulator surface when using the 6 heat range and that this has disappeared when using the 7. Nigel Lacey and others often use a cooler 8 heat range when racing. I also note that when using the NGK BR6HIX, NGK recommend an air gap of 0.5mm (0.019") that appears "tight" to me? Also, try another plug, just because it's new doesn't mean that it will spark consistently under compression.

I use a standard B7HS, gapped at 0.7mm (0.028") that is one heat range cooler and gapped 0.2mm greater than suggested by NGK in their current catalogue and I am using an Electrex Hall effect ignition single plug setup on the 450 MKIII.

Hope the gremlin is found sooner rather than later and that you can enjoy riding the bike, which looks great.

Good health, Bill

[A.I.M]

Thanks again for the advice, Colin & Bill.
I stripped the new carb: the pilot line was indeed gummed-up. I managed to clear it with acetone, a wee bit of copper wire and my lungs! This carb definitely doesn't have the screw-in pilot jet (the old one does have it). I'm afraid I don't have a magnifier/watchmakers loupe to inspect with.
So I got her started, warmed-up for a few minutes, then took her out on the road for the first time since 1981. I have some carb tuning to do, as the engine wouldn't run without the choke, was unwilling to rev, and generally seemed gutless.
Bill: the engine hasn't run with the old carb since 1981, so a straight slip-in isn't an option at present. However, I have ordered parts to get the original carb back in working order to see what difference that makes. And while waiting for those I'll give everything else a good check-over and get a cooler spark plug. Is your choice of plug gap just a personal preference, or is there some reasoning other than the 0.5mm "seems tight"?
The gearing is higher than I remembered, so that resulted in a stall, but restarted without problems and I had a sedate circuit of the local roads in first & second gear. It felt like the engine wouldn't tolerate third gear.

I'm smiling today!
Cheers,
Al

[Duccout]

Hi Al,

At least you have made progress. All the signs are that you have some sort of fuel starvation, possibly an air leak? I know that it is a pain, but if you have a degree disc I would check the valve timing and the ignition timing, just to eliminate them as the culprit. Mototrans engines have two timing dots on the lower bevel gears and it is easy to get the timing wrong.

Cheers,

Colin

[themoudie]

Aye Al,

Is your choice of plug gap just a personal preference, or is there some reasoning other than the 0.5mm "seems tight"?

My understanding, such as it is , is that with electronic ignition systems (you have a Sachse system fitted) there is plenty of "Snap, Crackle & Pop!" in the spark and increasing the plug gap gives improved combustion of the air/fuel charge. I have taken the following quote from the NGK website and given the link below.

Most experienced tuners know that increasing the gap size increases the spark area exposed to the air-fuel mixture, which maximizes burn efficiency. For this reason, most racers add high-energy ignition systems. The added energy allows them to increase the gap, but still have enough voltage to jump the gap.

Link: NGK_Gapping_&_Indexing_spark_plugs

Don't forget the newly built engine will be "tight" for at least the first 500 miles (800Km) and probably a for a wee bit longer. My routine is 20W/50 mineral oil for the first 20 - 100 miles (30 - 150Km), drain the oil and check for any "bits", metallic or otherwise. Flush the sump out with some aerosol brake cleaner, leave to evaporate. If all clear, then again fill with 20W/50 mineral oil for the following 200 miles (320Km) and repeat the process for another 200 miles (320Km). Hopefully, the engine should be bedding in nicely by now and you can start using the full rev range, so drain the sump and again fill with 20W/50 for the last time and after a further 500 miles (800Km), drain the sump and use either a 20W/50 semi-synthetic, 15W/50 synthetic oil, or oil of your choice, changing it at the factory 1,200 Km intervals. Personally, I am using a 15W/50 synthetic oil and changing it at 1,000 miles (1,600Km) in the 450 MKIII.

Hope you can sort the carb gremlins sorted out and get to enjoy the bike.

Good health, Bill

[A.I.M]

The carb parts are on order for the original, and a BR7HIX plug. Meanwhile I've removed seat & tank and gone over all the wiring. I don't have a dedicated engine earth wire, so I will look into whether I can fit one somewhere.
I'm not sure that I am competent to check ignition & valve timing, so I'm hoping that changing the carb will resolve the problems. I have not yet used gasket sealant on the rubber gasket between head & carb, so next time I'll smear sealant on both faces before assembling.
Hope to update again next week
Cheers
Al

[themoudie]

Morning Al,

Do not go smearing gasket sealant about on the gasket between the carb and the head, it'll just make a mess. If you MUST use gasket sealant, apply it thinly, with a small artists paint brush.

If the carb and cylinder head mating surfaces are true you shouldn't require any gasket sealant as the rubber gasket should be sufficiently flexible to make the seal. Is the "O" ring in good condition on the carb and the intake flange flat and not warped? Overtightening of the nuts holding the carb to the head can cause this problem. Check the cylinder head intake flange for the same uneveness, it should be level in all directions. Use the edge of a steel rule and a bright light behind it to see if light can be seen shining under the straight edge of the rule.

Depending upon the severity of the warping, if any, you will need to remove this before you can achieve an air tight seal.

Others on the forum may have their own methods/tricks when assembling the carb to the head?

An engine earth wire could make a difference, again other owners have their own preferences as to where/how they attach the wire to the engine. Mine is tucked up under the fuel tank between the head and the frame spine tube. The earth for the ignition, engine and rectifier are all wired individually to a common earth post on the frame beneath the seat, rather than relying on the frame to make the connections through the paint and greased fixings!

Hope you succeed, Bill

[blethermaskite]

Just to reinforce Bills advice re electrical earth points......do not trust any earth fixing point on your frame without inspection.....if the frame has been plastic coated or painted with modern '2 pack' paint it is essential to clean any earth point down to bare metal otherwise you will have a very high resistance connection which will manifest itself in all manner of strange electrical behaviour.
Cheers,
George

[graeme]

You can connect an earth cable to the clutch cable mounting on the crank case.
Graeme

[A.I.M]

Colin, Bill, George & Graeme, your advice is incredibly helpful, and I'm sure it will have the gremlins sorted out before long.
Messages received and understood regarding gasket sealant, running-in oil changes and earthing. I checked the carb and engine flanges for flatness, they are good (on both old and new carbs), I have the 7mm thick rubber gasket which should be fairly tolerant of imperfections.. I dismantled the (new) carb again and gave it another once-over with carb cleaner. All the galleries in the carb seem to be clear and no obvious problems.
Graeme, I had been pondering how best to fit an engine earth without disturbing the torqued engine bolts, and I came to the same conclusion that the clutch cable mounting would be easiest place to fit, at least until the next time I have to remove the head.
Cheers,
Al

[A.I.M]

Latest update:
- The old, original carb has been refurbished with all new internals, and installed in place of the new carb. I made sure that the float valve closes at the same height as the original.
- I installed an engine earth directly between the clutch cable mounting bracket and the battery negative
- I changed the plug for a NGK BR7HIX with 0.6mm gap
The engine starts easily enough, and after a few minutes of warming-up it is possible to withdraw the choke and get a steady idle. However despite the improved idling, it hesitates and ultimately dies when trying to rev the engine. There is no longer enough engine power to get the bike moving.
I should mention that with both carburettors there is a good flow of petrol when depressing the tickler.

Two different carburettors, slightly different (but similar) symptoms - frustrating!

I had hoped that it wouldn't be necessary to check the valve and ignition timings, because I am not convinced that it is within my capability, but it seems that I am going to have to do that.

Colin: you mentioned that "Mototrans engines have two timing dots on the lower bevel gears and it is easy to get the timing wrong." - do you (or anyone else) have a picture of how the dots are intended to line-up?

Cheers,
Al.