M110 D Jet 1975 strange starting problem driving us crazy?

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Hi all been working on this w114 1975 M110 D jet and now up to the ignition. Ok so the problem:

When we first try and start (hot or cold) she turns over for about half a min as per normal but doesn't catch then it starts to like bog almost like misfire so we stop and try again & on second attempt it starts in a split second like nothing wrong?

We tried this over and over and always the same, first attempt no go it will turn over normal but not catch and then starts to bog down like misfire and or low battery...we then turn off and immediately try again & turn ignition on again and bang she starts first shot , it has never started on the first attempt even if we hold the ignition for a bit longer, it only starts to bog down like its misfiring??

We Went to a few places and made the following changes as per recommendations but still the same problem:

was told starter could be faulty so had it tested and it performs as it should, new battery, new clamps, new earth wire & connection, new spark plugs (wr7dc), still the same.

the last guy thinks it could be a leaky cold start valve or may need new positive wire??

Anyway anyone else have a similar problem or have any tips to follow up on? thanks