MGTF performance issue

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MGTF performance issue

Post by tedjan » Wed Nov 27, 2019 11:26 am

Hello Group,
This is a follow up to an earlier post in which I requested assistance on installing pistons in this TD block.That job is happily done.This engine is .060 in..oversize at the bores but was rebuilt not too long ago.however the carbs were very rich which I believe washed down the cylinders and prematurely wore out the rings.Cylinders measured out correctly new rod bearings were installed.Piston ring gap was within spec and we had a new set of valves and springs and guides installed and valve seats refaced.Engine no longer smokes and compression is an even 145 lbs.across all cylinders.New spark plugs,Champion na8.This is a TF head.
Distributor has Petronix ignition and we replaced the stretched out springs for the advance.Timing is set at 8 degrees btdc @idle.Cap,rotor and wires are good.
This engine has a B and G race cam and we were advised to adjust the valves to .015 in..,hot.
We rebuilt the carbs with rebuild kits from Moss.Used standard needles for the TF carbs,GJ.Engine idles smoothly and accelerates properly in the shop.
The problem:When driving on the road,if you accelerate normally,the engine breaks up and will not accelerate.However,if you are very gentle on the throttle pedal and give it gas slowly you can accelerate smoothly throughout all gears with no performance issues until you reach high gear which needs more throttle to accelerate and then hesitates slightly.
Book calls for a float adjustment of 3/8 in.but carbs tended to flood,so I set them at 7/16 in.and they no longer flood.
Not sure where to go next and would appreciate any advice.
Thank you and Happy Thanksgiving.

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Steve Simmons
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Re: MGTF performance issue

Post by Steve Simmons » Wed Nov 27, 2019 11:38 am

Definitely sounds like a fuel issue to me. Have you checked the flow of the pump? If I have my facts straight, it should deliver 1.3 US pints per minute and hold a pressure of 1.5 psi. I suspect something else is going on though. If you have an A/F meter, see if the AF ratio is staying within an acceptable range. I've had carbs WAY out of adjustment before without major issues, I wonder if there is an intake leak somewhere? Don't forget to check the core plugs in the ends of the intake manifold. It's rare but they can leak.
1949 TC8975 / XPAG 9609
1949 TC9849 EXU / XPAG 10507

Duncan M
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Re: MGTF performance issue

Post by Duncan M » Wed Nov 27, 2019 2:02 pm

Bets are on intake leaks at throttle spindles.

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stephen stierman
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Re: MGTF performance issue

Post by stephen stierman » Thu Nov 28, 2019 3:44 am

You may want to try a richer needle and see what happens, it does sound like a fuel issue.

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Graeme Louk
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Location: Sydney, Australia

Re: MGTF performance issue

Post by Graeme Louk » Fri Dec 06, 2019 5:22 pm

Good Morning,
Yes it is possible to get a result through trial and error however it’ll take time and requires a skill set that is fast becoming a lost art/ trade.
I recommend you get your car tuned on a dynometer. Find a shop with a Dyno and a distributor machine that does a bit with old English motors so they are familiar with SU’s. Dyno’s aren’t just for race cars. Get it right the first time and subsequent tune ups are a cinch to do at home.
With the technology available today it’s possible to get it right fairly quickly so you’ll be out enjoying the results immediately. The relatively small investment (yes it costs money but consider the cost of a dyno) is well worth the pleasure of driving a well sorted car.
If you choose the trial and error method it’s possible to get an acceptable result but laborious. As far as air fuel ratio goes make certain that you don’t take it at the common exhaust without having the front two cylinder exhaust temperatures the same as the rear two. It’s possible to have one carb lean and the other rich. An Infa-Red thermometer is acceptable for comparing the exhaust temperatures (be aware of airflow over the manifold). Or better still fit a Dual EGT with one sensor in Number 1 and the other in 4 exhaust.
Graeme Louk

Clifford Jay Lockrow
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Re: MGTF performance issue

Post by Clifford Jay Lockrow » Sun Dec 08, 2019 6:29 pm

Have you checked all the fuel lines to see if there is a blockage anywhere. I once had a tiny piece of rubber from a fuel line that was caught in the top of one of the SUs. The result was the me as you are having.

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